If everyone was spread out onto different instances, and communities were based all over the fediverse, the decisions of one instance would be less impactful.
If people would share the idea of the fediverse instead of saying "yeah reddit suck, go to this website instead", this would put a dent in it.
But since the concept is so alien and hard to describe, people find it easier to just share the site, and since that game keeps being recommended, and since even if they know about multiple sites working together, even those people are going to go to one that has a friendly name, so this is what happens.
I'm only not on it because I like picking less popular things in general, so I actively avoided picking what seemed to be the default at the time.
Also I believe it would help if the sites/instances had a way of distinguishing themselves more and communicating their differences. Even most of the instances' intro or about pages are mostly saying something like "hey I'm a general use instance, with mostly this language, pick me!"
Which in and of itself is fine, but it seems most of them are general use, so people have no basis for picking one. They may figure out different reasons to like one or the other along the way, but once they pick one initially, I don't think most people make another account.
I haven't done much of that either, except for making one my dedicated NSFW account and this one, but I plan on making at least one or two more just in case of downtime, or even to separate genres of content.
The problem is that Lemmy is not federated. You can't click this link /c/books and get the whole fediverse book community. Federation dies right there.