I don't have direct experience with either Helix or Kakoune but after only a few minutes tinkering around, I can see one big reason: In Helix, most of the basic commands seem to be the same as vi. Whereas I understand Kakoune inverts the action/movement paradigm of vi. Maybe that's a more sensible design, I don't know. I didn't check to see whether or not the key bindings were similar but at that point, it's rather moot.

I've been using vim for 25 years, my muscle memory isn't going to tolerate switching to a whole new text-editing "language" at this point. But I could perhaps learn to live with a new dialect.