Can't speak for others, but my decision to stick with lemm.ee stemmed from seeing how active the owner is, the minimal defederation and the bot & moderation policies meaning there are minimal bots, and I've yet to currently see much spam issues, which is nice.
Also helps that the owner Sunaurus is an active contributor to the Lemmy project as a whole, so we get patches and fixes relatively quickly.
When I tested different instances lemm.ee was quite small compared to the big ones. The main pros are technically fast and stable. It also federates comprehensively.
Edit: also remembered that it hasn't had any insane admin/rule situations that I've noticed
They got aome of the most traffic during the migration, so it pulled itself off the instance list page i think for regiatration so that people would join other instances leveling the load
They closed sign ups during the early days of the migration up until recently. I just missed the moment they're open again since I'm not interested in large instances.