Technically, it doesn't. In practice, using a unified and polished interface like Github make things easier.

I feel like actually creating a fork of vim that is viable for the community is a much higher bar. If that first bar stops somebody, is it likely that they would have cleared the second bar?

Vim appears to be Mercurial while Neovim is Git. I imagine merging is a lot easier when you are on the same VCS :)

You can use hg-git[0] to push to Git from a Mercurial repository. Then it's basically just like working with another Git repository. It does mean that you need some way of mirroring the two. I've never tried it, but it looks like you can use git-hg[1] to use a Mercurial repository like a regular Git remote.

[0] https://hg-git.github.io/ [1] https://github.com/cosmin/git-hg

I don't think there is going to be a lot of merging behind done so it probably doesn't matter.