I learned the reason why Dom DeLuise lent his voice to so many Don Bluth movies was because he was actually contracted to do so; this is why (with the exception of A TROLL IN CENTRAL PARK), we don't hear his voice in any later Bluth movies, because his contract was up by then.
Makes me wonder if some of the other actors who lent their voices to Don Bluth movies were under a contract as well, like how Charles Nelson-Reilly often played the villain's lackey.