As someone who only recently joined Lemmy (as a result of getting booted from Reddit) it ultimately comes down to it being bigger.

You can talk regularly about series have been over for a decade. Just about any niche interest has a vibrant community. The reality is the average person doesn't care about it selling our data, putting a fingerprint on our gear one step above spyware, it being overrun with bots, every level of administration being dominated by megalomaniacs. If you just want to look through some stuff you're interested in while you're bored, it serves a purpose that lemmy unfortunately can't at it's size.