Rust was originally built with green threads. The RFC that proposed removing them has an extremely detailed explanation of why the change was made pre-1.0: https://github.com/rust-lang/rfcs/blob/0806be4f282144cfcd55b...

I think this has proven itself to be the right decision. It's one of the main reasons that Rust works so well alongside other languages—there's very little default Rust runtime that complicates interop. There are also lots of programming scenarios for which green threads are not the right tool, and Rust accommodates those.