Actually, We killed the legion leadership and put the entire thing in a power vacuum there are still PLENTY of demons left, They did go on a multi planet genocidal join us or die kind of spree afterall.
As for Draenei the reason why is the draenei are hard opposed against the legion they aren’t about to bathe in the demonic power that makes a demon hunter and endure the binding ceremony.
Orc Demon hunters would make sense however.