It's one thing to contribute a couple of PRs/patches, a very different thing to spend a large part of your free time on the project for years (which is what Bram did to make vim what it is).
These projects are no joke and a few hurray contributors not lasting more than a couple of months won't cut it.
Those same contributors that remained on vim instead of neovim didn't put much effort into vim9 ecosystem either. I can't imagine them being willing to maintain all of vim's baggage when literally nobody writes vim9script. Classic vimmers tend to stick to the old vimscript.
As you said, contributing a few patches to classic vim is one thing, but becoming an actual maintainer of the behemoth that comes with a ton of useless baggage is a whole another thing. Maintaining a programming language no one uses.. sisyphus would be proud.
Vim is a marquee project, tons of hackers will want to contribute because it's cool or because they want to have that name on their CV.