Why doesn't the enemy's afterlife resolve first?
Yes, on your turn your opponent's Afterlife triggered ability gets put on top of the stack, so it resolves first, and then your Dreadhorde Butcher's triggered ability resolves.

However, that isn't enough to kill the newly created spirit token with the Butcher's ability. You choose targets for abilities when you put them on the stack, so the Butcher's ability needs to target a player or a creature or planeswalker already on the battlefield at that time. This happens before the Afterlife ability resolves or is even put on the stack, so the spirit token won't be available to target.