If Judaism is the oldest Abrahamic religion, why does it have so few followers compared to Christianity and Islam and why by such a large margin?

Judaism is what's called an ethnic religion, rather than a universalizing religion. Ethnic religions generally don't seek converts. There are accounts of forced conversion by Jewish fundamentalists in the Second Temple Period, but even then it tends to be of peoples they regard as ethnically related and 'fallen astray' rather than people they see as out-and-out foreigners. Ethnic religions are regarded as a pact between the Divine and a specific set of people - and outsiders are either viewed neutrally or negatively coming (or trying to come) into the faith. As such, ethnic religions generally only propagate by birth or, less often, intermarriage. They don't want outsiders flooding into their private ethnic pact with the Divine, and, indeed, sometimes regard it as religiously offensive.

Universalizing religions, on the other hand, are quite explicitly peddling a view of the world that does not, theoretically, have ethnic or cultural borders. Christianity and Islam, both universalizing religions, desire Asian Christians as much as African or European Christians. Theoretically. There is a... great deal of difference between what is taught and what is executed, but in general you can assume that universalizing religions like those more or less always want a person as a convert, regardless of their background. The pact with the divine is regarded as personal, rather than ethnic, and the divine regarded as largely impartial to all the minute divisions of mankind.

They believe that God wants us to use our free will, which means that proselytizing isn't kosher (har har.) Where Christianity and Islam have spread pretty aggressively (often literally), missionary work or let alone forcing people to convert is anathema to Judaism

As a Jewish person myself its for a few reasons:

1. We don't forcibly convert, we also actively convince people not to convert (I mean using every argument possible to convince people). Jewish Philosophy states that if someone is holy enough to go out of their way to join they're holy enough to be Jewish (the conversion process is long and if it's not sincere they aren't accepted).
2. Jewish power structures have always been contained, in Israel for example Jewish religious leaders have little power.
3. Judaism has a long history of expulsions and being massacred, also forced conversations away from Judaism
4. Due to Jewish fundementalism its common to see people stop identifying as Jewish. For many people the Jewish community can be extremely hostile and unwelcoming, a few examples include: women, Neurodivergant people, Queer people, people without a Jewish education, people who dont fall into traditional gender roles/expressions, etc

Judaism tries to disincentive people to convert while Islam and Christianity both try/have tried very hard to convert people, often at the tip of the sword.

One thing no one mentioned yet.

Christianity really spread once it became Rome's state religion, and Islam was always about expanding.

Once the Jews got kicked out of their homeland they were stateless people. Pretty hard to go against local religion when you're a guest with no army.

I am not a historian but my bet would be that it's ultimately a political question.

Christianity was the religion of the late and medieval Roman Empire, and later of the western European colonial empires.

Islam was a religion that became a caliphate and a series of empires afterwards.

There has never been a Jewish empire, and the Jewish kingdoms (Hellenistic Israel, the Khazar khanate, Beta Israel) have been small, non impetial and relatively short lived.

So in addition to what pretty much everyone else says, you should note the conditions of emergence. Judaism not only doesn't seek converts; the people it considers its followers were a subjugated people. Not in a position to spread anything. And then after the Romans drove them out of their land a couple centuries BCE, they were basically the world's biggest refugee population. Again, not a situation that helps proselytization even if they wanted to do it.

Meanwhile Christianity managed to become the state religion of Rome and basically all of Europe after the collapse of Rome. Added up that's more than 600 years as the state religion of a global superpower (or superpowers) that actually cared to spread their religion. When you also remember how European colonization was a thing, you basically have three continents' worth of Christians and significant populations elsewhere.

Meanwhile Islam came and united the Arabs under one banner. These people, who were only concerned about writing poetry, trade and killing each other for a while, suddenly had both the means and (material and spiritual) incentive to expand. With the two extant superpowers greatly weakened, nobody could do anything about their light cavalry and sheer zeal (remember the Syria crossing?) for a while. You know how the Mongols were united by Genghis Khan and conquered basically everyone? Yeah that's basically how it went. Muslim merchants, with their country now the wealthiest in the world (sans China I don't know much about them in that time period) went everywhere and had a tendency to convert trade partners, which got Islam a foothold in places Muslim rule never reached. BTW here I'd like to correct a common mistake: Islam usually didn't force people to convert. However, Islam was still the state religion and at the top of the socioeconomic ladder, and depending on time and place non-Muslims had significantly harder lives than Muslims. These factors fueled conversions and in a few centuries Islam was the majority religion in these countries.

The Holocaust may have played a role

Part of why is that Islam and Christianity spread to other cultures, often with force. Judaism doesn't give heaven points like they do for converting someone.

It's because new religions always cherry pick the parts they like of old religions and then eventually replace them entirely. Judaism itself was just made up from parts of Zoroastrianism and Hellenism and Babylonian beliefs.

It's all bullshit anyway so it hardly matters. Religions change as needed to meet the changing wants and needs of the people who follow them. A Christian from the time of the crusades would probably be horrified by what is considered to be Christianity today.

For one thing, they don’t proselytize like other religions, and they’ll actively try to deter you from converting.

Looking at religion purely as a means of comfort for people who are afraid of dying, I wouldn't think Judaism is very popular compared to Islam or Christianity because afaik, Judaism doesn't have a clear way to heaven.

There were many streams of Judaism in ancient times, and they were not as closed off as they are today. Jews converted other people. The major distinction between Judaism and that of Christianity and Islam is the sense of universalism. The concerns of Jews are mostly toward Jews. Not outwardly toward non-Jews. It is a covenant with God. Christianity and Islam proselytize for universal reasons. For Christians, to save the souls of people against the eternal damnation of the original sin and to seek eternal existence with God. For Islam, it is to expand the community of Muslims, and in a similar Abrahamic eschatology, to prepare mankind for the end times, which strangely involve Jesus. In belligerent times, more Muslim recruits for war.

Christianity was a religion for the lower strata of society. In a world, where poverty was considered normal, and frowned upon. Poverty, disease, and oppression were considered a deserved affliction. It was a world without compassion. Entire economies were run by slavery and a Roman, saw it as his duty, without any doubt, to discipline his rebelling slave, because of what is at stake. Since there were more poor people and downtrodden people, Christianity spread like wildfire. It was not a religion just for the elites and the rich, whose gods were only concerned with superior things, even pettiness. This type of morality is the basis for Marxism, feminism, communism, etc., because the New Testament is about Jesus being against oppression, regressive taxes against the poor, and treating people equally, and against corruption. The poor in Christianity have a special purchase on the rich. For the Romans, who were looking for a religion to unify the empire, saw Christianity as the most marketed religion, and made it a state religion for that purpose.

I think Islam started as a war booty army that expanded control of ceded territory by the bankrupt Eastern Romans, who sacrificed much to lay a technical defeat on the Sasanian Empire. The Saracens, who were anti-Roman as pagans, and even as Christianized tribes afterward, they were known as thieves who robbed the caravans of the Eastern Romans. That culture was palpable to the Turks. It is important to note that the evolution of Islam involved different factions of people. For the example, the Turks, the Saracens, the Persians, etc. The Qur'an is a collection of heretical literature and theologies that are not mainstream Christianity today, but were in significant communities under the thumb of the Eastern Roman rule, who had a policy of orthodoxy. What was to become Islam, was originally anti-Trinitarian Christianity. The Qur'an contains mostly a Christological polemic against other Christians. If you were anti-Roman, how fitting it is to be against Trinitarian Romans by being non-Trinitarian. I digress.

For the Jews not having any type of universal mission for mankind in general, this was a disposition of being dominated by those religions that were universal.