The brand promise of Reddit was pretty simple—it was the “Front page of the Internet”.
It did not get popular because of the sub-communities or that there was a sub for everything ( at least not at first ).
Reddit became a thing because it was a single destination that aggregated and curated interesting content from the web that “interesting” people could comment on. If you were only going to make one stop on the Internet, it could be Reddit. Uses could share the main URL by word of mouth and new users would get the same experience. As content grew, Reddit became high ranking in search results.
Lemmy does not really offer the Reddit experience to a new user. New users do not want an offer to find an instance or create one, they want to experience the content, get addicted, and come back.
The closest Lemmy has right now to early Reddit is Lemmy World but how do new users know that? Actually, I guess old.lemmy.world is the closest. :)
I think that won't be as big of an issue in time. As Lemmy grows, eventually people will be exposed to it and other services on the Fediverse and will be more likely to have an idea on how to get started, or at least find good guides.
Remember that pretty much everything on computers requires some instruction at the beginning. The advantage that Reddit and other software have is that people have (and continually are) already taught how to use them.
It's a similar situation to Linux vs. Windows. A lot of distros on Linux are actually more user friendly and easier to learn than Windows - the issue with getting people to try Linux is that they already know how to use Windows and most people hate learning new things