Just to be clear, I'm not saying there should only be one scheduler *implementation* in existence
But having said that, I can't see much reason why you would need to have more than one scheduler implementation.
Multiple event loop implementations are necessary because async I/O needs to be done different ways on different platforms.
But the scheduler we're talking about is all pure Python. If the interface is well known and universally used, and there's a good implementation of it in the standard library, why would anyone want another one?