One thing that hasn’t been mentioned yet is that Lemmy simply is older and better known, and both leads to more communities being created there, which then leads to more content being created there, even if it’s kbin users creating that content.

While kbin was only launched a few months ago (April?), Lemmy launched in 2019. Both didn’t get much traction before the Reddit blackout, but for the Lemmy users there was more time to create communities which already were there then when kbin users started looking for them. Even those communities that were created on both had a chance of already having a parallel community on Lemmy that possibly was more active (i.e. had at least some posts), so future posters might have decided for the Lemmy one more likely.
And for the “more popular” thing: Lemmy usually was named as the alternative to Reddit, so many Reddit users came to Lemmy, looked for communities they were missing, and created them, possibly before learning of kbin and eventually moving there themselves.