Three reasons. First, we don't control the kernel and we don't want to make assumptions about thread spawning being cheap on every OS. Second, it lets us implement work stealing, which is a proven method for dynamic parallelism. Third, it lets us do some operations such as RPC from task to task entirely in userspace with no trip through the scheduler or OS kernel.
Good points. A counterpoint for #3 is that you could do kernel bypass RPC by installing a driver, but Rust developers probably don't want to write all those drivers, and Rust users wouldn't want to install them.
Work (task) stealing is very compelling, and a little paradigm shift is no bad thing. If Rust or any new systems language stands a chance, it should aim high and not too close to the past.
