The original founder's list was Dave Wiskus (current CEO), CGP Grey, and a guy behind Kurzgesagt, but yes, the latter two distanced themselves pretty quickly.
I don't think there's a single even semi-popular edu-tainment channel that wasn't at least approached by Nebula, but of course not every creator is interested. If I had to guess why, I'd say it's a tradeoff between Patreon/Nebula. You can have both, but you can only prioritize one of them, making the proposal far less interesting to those that already have a sizeable Patreon income.
Sure, as long as you don't swear, play even a a couple seconds of copyrighted music, never say any of the "touchy" subjects that would earn you an info box below the video (could be anything from flat earth to covid), and a dozen of other arbitrary, constantly changing rules, YouTube is great on its own. Certainly better than any other popular platform.
However a lot of channels available on Nebula also use it as a place where they don't have to deal with those sometimes nonsensical rules, as a place to publish more "uncensored" versions of their videos. Sometimes they're as simple as not bleeping out the word fuck, sometimes they contain an extra section that would get their entire video demonetized on YouTube (example: showing a piece of art with nudity in it, or pretty much any blood), and sometimes they use it to cover topics that they know YouTube would instantly demonetize (example: pretty much any discussion around armed conflicts).
I don't think there's a single even semi-popular edu-tainment channel that wasn't at least approached by Nebula, but of course not every creator is interested. If I had to guess why, I'd say it's a tradeoff between Patreon/Nebula. You can have both, but you can only prioritize one of them, making the proposal far less interesting to those that already have a sizeable Patreon income.