Preempts particularly major suit preempts work.For making a ♦ slam, you atleast require ♥K & ♣Q {assuming preempter is having the ♠A} easy ♦distribution.So after 4♦ bid, partner will naver be able to show those features & only can raise into 5♦ & then it will be a gamble to bid slam.In that case it is better to bid 5♦ to give partner a chice to bid slam depending upon his hand & judgement.On the other hand slam in♣ requires 4 carder ♣Q & easy ♣distribution.Now it is a choice between pass,5♦ & 4♣.Difficult hand to bid,my choice will be 4♣-still a gamble :(