Funny, Kbin’s UI design and better performant feel were major reasons why I switched over from Lemmy. It reminded me of old.reddit.com, which I feel familiar with. Shows that it might be a matter of personal taste. Or maybe you experienced a different kbin design (there are still major updates every week, the last one just yesterday).
One additional point that also influenced my personal choice was the political stance of the Lemmy developers (edit: removed my reference to these rumors, because now I‘m uncertain how much of it was true). However, that might not be that big of an issue in the open source environment than it might appear at first.
Mastodon integration feels unpolished, but I‘d rather have it than not have it.
I think that voting on kbin is private now, by the way. Only boosting (retweet) is not private. (Edit: I was wrong. See below.)