r/Redditalternatives describes the various reasons that people decided to abandon Lemmy and go back to Reddit:

• lack of content
• too difficult to figure out how to use
• it’s “the place that tankies built”
• too toxic, extremist leftists piling on anything not supporting their dogma hard enough, or sometimes just “too political”
• overall they do not feel welcomed. Tbf many on Reddit are centrists or even conservatives, and quite toxic themselves.

Notably, PieFed provides solutions to many of these e.g. the sign-up wizard that subscribes people to communities based on their indicated interests.