No evil mobs spawn in my world, only they spawn in the nether

No evil mobs spawn in my world, only mobs spawn in the nether I have no cheats activated nor have I used commands, I have it in normal or sometimes difficult mode and when the world just started if there were evil mobs but not now, only in the nether. I play in multiplayer and I am the operator of the world and the other players have the same problem.