I got a response from them on Reddit:

We didn’t know which platform would take off, and we were nervous that because Kbin and Lemmy are so similar one platform might shut down in some sort of consolidation down the road. Also when we made them, each had very serious drawbacks for our media (Lemmy needs a lot of clicking to access the media, while kbin turned any media that wasn’t in a 3:4 aspect ratio into a funhouse mirror.) So each of us took a community and somewhere down the line we’ll re-evaluate.