Totally different implementations of activitypub servers, for one thing.
Kbin/mbin were also designed to try to support both interfacing with micro blogging (as in Mastodon and a few others) and with Threads (as in Lemmy and a couple others).
It's pretty good overall, and I started on kbin, but I've found myself using Lemmy instead for a while, mostly because of better clients on mobile. And a handful of technical/design issues on kbin that were annoying me (although some have been improved since)