As far as I understand, green/user threads were on the table in rust team initially, but were sorted out because having green threads also means there is a runt

As far as I understand, green/user threads were on the table in rust team initially, but were sorted out because having green threads also means there is a runtime managing them which rust tries to avoid. What I do not understand is where the principal difference with async lies: it has no runtime, but you have to bring your own for it to work anyway in form of e.g. tokio. What rust goal conflicts with a similar solution for green threads?