They already made the mistake picking a niche language twice (first their own language, then Swift as a cross-platform language), why would you want them to make it a third time?

I've dabbled a bit in Ada, but it wouldn't be my choice either. It's still susceptible to memory errors. It's better behaved than C, but you still have to be careful. And the tooling isn't great, and there isn't a lot in terms of libraries. I think Ladybird also has aspirations to build their own OS, so portability could also be an issue.

Probably contributing reasons? I imagine over time they will have a lot more Rust contributors than D or Ada.

Unfortunately a really good question gets downvoted instead of causing a relevant discussion, as so often in recent HN. It would be really interesting to know, why Ada would not be considered for such a large project, especially now when the code is translated with LLMs, as you say. I was never really comfortable that they were going for the most recent C++ versions, since there are still too many differences and unimplemented parts which make cross-compiler compatibilty an issue. I hope that with Rust at least cross-compilation is possible, so that the resulting executable also runs on older systems, where the toolchain is not available.