Since Kbin was still at what Ernest refers to as a proof of concept stage when the project reached peak popularity, and I think at the time (and probably still) it was really not particularly easy to run an instance of. That's probably part of the reason why so many of the early Kbin instances died off.
According to FediDB, the biggest instances other than kbin.social are Karab.in and Kbin.cafe. Karab.in is run by Ernest who also runs kbin.social - it's just the Polish instance. As developments are happening fast (see !kbinDevlog), it's to be expected that instances not run by Ernest lag a little bit behind in development.
It seems the administrator of kbin.cafe is @barista. I'm not sure how actively maintained it is.
Considering the fast pace of development, I think it makes sense for the time being to either use kbin.social or an mbin instance.