Why There are only humans for service on darth vader's ships'?

Recently I watch Star wars Episode 4 and 5 and I notice that On Darth Vader's Ships' (Deth Star, Executor) and other ships, there are only human species as a service (worker - Crew). There are no alien ( other species ) are available as crew. Is there any special reason behind this ?

In Episode 5, there are some aliens was there on deck but they are all bounty hunters.