I think it mainly comes down to the project landing page being more friendly and the UI being more polished.
The landing page of join-lemmy.org doesn't show what the website looks like. The only screenshots are of code and github. That section is geared towards potential instance administrators, not potential users.
For my own projects I’m trying to build things I actually want. Tools for myself. But now the hard part becomes identifying a tool you wish you had, and scoping it down enough so that it’s appropriately sized for a new language. Tricky to approach the task from two ends.