I don't want to list specific contributors here. We often have no information on why people left the project - there might be all sorts of personal factors. I will say that my broader sentiment - that Rust's momentum has slowed, that it's not delivering on its commitments in a timely way, that there are concerns about it's ability to deliver in future - has been expressed publicly by high profile past core contributors:
https://github.com/rust-lang/rust/pull/96709#issuecomment-11...
I don't think Rust should tackle any ambitious project to rewrite the compiler while these basic concerns remain.