Since both threading models were supported and compatible throughout IO, concurrency primitives and other runtime bits, the design of both was considered to be

Since both threading models were supported and compatible throughout IO, concurrency primitives and other runtime bits, the design of both was considered to be constrained too much to get either model where it should be. The decision was made around https://github.com/rust-lang/rfcs/pull/230 https://github.com/rust-lang/rfcs/pull/230 but I'm not sure whether there's any important prior discussion not linked from there, the topic has been kicked around for a bit.