Just more people knowing about it. No one knows it exists.
Also, more normie subreddits. A lot of people join reddit for specific communities. The most active Lemmy communities are political or tech focused. Art communities like imaginarycityscapes or nosleep and fan communities for shows and movies draw in a lot of people. Lemmy's userbase is made up largely of people who are able to comprehend what federation is, which is limited to people who understand what a server even is, which has an impact on the type of content.