Gnomes and humans and etc. didn’t go run off with Illidan to be Illidari. Elfs did. If there were no factions I’m pretty sure only Night Elfs would have been DHs, but yeah, making other races DHs doesn’t make sense without a massive bastardization/retconning of the lore. Which WoW is totally game for - just look at Shadowlands. So I guess we never know with this writing team.