Rule

there are far fewer but there are some (fedidb list), notable ones include karab.in (polish instance) and artemis.camp (special instance that's used for Artemis app development, also the one that @pewg7 uses)

as for why there're way more lemmy instances, my opinion is that it mostly comes down to:

• lemmy comes first -- first lemmy release dates to around 2019 while kbin's first commit is at 2021 and apparently it was not until sometimes in 2023 when the main instance came up
• had time to be tested and build its own reputation, and overall looks more matured and stable compared to kbin (I've seen quite a few opinions that although they find kbin nice, it's still needs more development to be more usable/comparable to lemmy)
• using more modern tech stack and organization -- rust backend, inferno (react-like) frontend, split frontend and backend design; vs using php which is perceived as dated and it seems almost a rite of passage to make a "haha php bad" joke at least once, and I don't think using Symfony is going to save that
• having a working API implementation -- when the reddit thing happened it didn't take long to see new lemmy app developments exploded like crazy, some were from those who made third party reddit apps and they had rework/adapt their apps into talking lemmy, and then there's Artemis where kbin didn't have an API implementation yet when development started, and the earliest version was resorting to scraping kbin's web for data (from Artemis FAQ, second point)
• (partial information, take this with a bottle of salt) supposedly more healthy/inviting for developing/contributing -- I cannot say for lemmy, but kbin's development seems to be slowing down as of late and stopped to a screeching halt several weeks ago, apparently the head dev got some big irl problems and so they basically went missing, and things starts to fall apart (one instance is that the main instance went down for like half a day).
  • in fact it went so bad that one of the developer/contributor decided to make a fork (mbin, fedidb list) and some of the contributors/instance operators has since moved their instances/efforts to mbin instead
  • one instance of note: fedia.io, it used to be running kbin for quite some time, but things starting to fall apart, severe performance degrade and errors to the point that the instance owner was considering to shut down instance but has since migrated to mbin where the situation seems to be going far better, enough for the owner to retract the shutdown.

(also first serious post hope it goes out well)