The problem is that said GNU support (which AFAIK is the one everyone interested in the language uses) is contributed by ... Adacore. They can decide to stop doing that should they wish to do so. I might be out of date, but their Ada compiler is the only one that's up-to-date, targets multiple platforms, and can be used for free. So they have a sort of de facto vendor lock-in.

Also, if you have to recreate a collection of lib from scratch, I have the feeling people would rather contribute to a newer language in the same (or same-ish) space doing just that, such as Rust -- which I think is what is happening, even if Ada still has some distinctive features.