Lack of awareness of Kbin I think. Also the monthly average users are the same, but lots of communities were created on lemmy instances.
Lemmy is older and I think when people thought a million people had signed up they thought that was the place to be. But it was all bits and the two are similar in terms of users, although most Kbin users are on Kbin.social at the moment.
I've tried both and I prefer Kbin. I like the interface, I like the mobile PWA app, it just feels more modern and polished compared to lemmy. But it's just a preference thing they're largely the same.