But it makes sense to include it here since kbin and lemmy are both link-aggregators and both part of the fediverse. From lemmy.world you see beehaw.org in the same way as kbin.social. No important differences. Same can not be said about Mastodon.
The issue is that the page where the screenshot was taken from doesn't allow to list both Lemmy instances and Kbin instances at the same time. You need to take two screenshots to show both services, or to use another listing source that shows all services you want to be shown, and I don't know whether it exists or not.
Sometimes it is, say here: https://fedidb.org/current-events/threadiverse
But you might as well say why isn't Mastodon included? Kbin is a fundamentally different platform and architecture, even if the two are largely compatible (as is Mastodon, to a slightly lesser extent with Lemmy).
It is perfectly reasonable to look at the growth of Lemmy, think of it like quoting use of one flavour of Linux - Linux uptake overall is also interesting, but not the same.
? You can search for Kbin magazines & subscribe to them via Lemmy, as well as post to them/reply to posts that are federated and the like. There have been some issues/delays in federation between the two, but not too different from those between Lemmy instances, unless something has recently changed with Kbin again in this regard.
All good! There were some initial issues with Kbin/Lemmy federation, at first I think it was that it wasn't enabled just yet and later it was related to their configuration of Cloudflare or some CDN, I think, to help in handling the traffic, but it was eventually resolved.