It's nice people, the culture of Lemmy, and the amount of users
On reddit, if you wanted to chime in on a thread that was popular enough to reach your feed, it was probably too late to make a comment that would stand out, since the people who comment on it early would get the upvotes, reach the top, and drown out your input.
Here at least, the comment sections, number of users, and the way "Hot" is sorted allows people to feel like their input matters, rather than just trying to make short quips to farm the most karma. The lack of a karma system or comment/post awards also helps this, as people aren't as incentivized to just farm upvotes.
And of course, the bulk of Lemmy's platform as of right now is built on people who left Reddit because they cared about their communities, and had strong opinions on how an online forum ought to be fairly run, leaving the more apathetic users behind. Naturally, this means most of Lemmy's users care about their community, and share that common bond.