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
lemm.ee itself also doesn't allow porn but that's a "don't post it with your lemm.ee account" type of rule. I think it also gets filtered out of the all feed, but you can definitely subscribe to stuff over on lemmynsfw.com. Long story short porn is a bugger to mod and the legal situation when hosting (and uploading an image with a lemm.ee account would imply that) is never trivial, either.
Somewhat relatedly, image upload limits are also rather strict, 100kb, to avoid that becoming a resource hog.
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.
Thanks. Looking at the admin's profile, he seems to be very reasonable. I especially like this comment he made:
"Not sure if I actually have to justify removing soviet and/or nazi symbolism, but I have lived my whole life in a country on the border of Russia. My country been ravaged by both Nazis and Soviets in very recent history. I want to be clear that while I think criticising capitalism is totally normal and necessary, I am also extremely aware of constant Kremlin propaganda. Propaganda which is spread both in their media, but also crucially in online channels. Propaganda which includes things like downplaying horrifying actions of the soviet union, portraying their “enemies” (the west) in the worst possible light, and mixing in soviet symbolism wherever possible. I have no intention of allowing anything which resembles this propaganda to be spread through lemm.ee. If banning offending users does not work, then further steps will definitely be taken."
History fits the majority of Eastern European countries but yes as said they are Estonian. Actually, now that I think about it can't really think of any Eastern European countries that it doesn't fit at all.
GitHub sponsorships are a way of donating to programmers. In addition to hosting lemm.ee, sunaurus has made significant contributions to Lemmy's codebase. (In my opinion, he is by far the most skilled programmer working on Lemmy.)