I think the reasons are economic really.
1)Having Hitler be a leaderhead in the game would make the game illegal in Germany, cutting out a major consumer base.
2)Having Hitler be a leaderhead will piss off alot of people, and further cut out large demographics from the consumer base.
As to Hitler being too evil to be in the game, utter nonsense. Hitler was a bad man, tis true, and so was Moa, Ghengis, and Stalin. All mass murderers, all genocidal, killing millions. Yet Hitler is not in the game for economic reasons based on politics. And frankly if I were calling the shots at Fireaxis, I'd do the same, even though I personally think the game would be more true to the Civ ceries to include Hitler. However from the companies standpoint, it's not worth it to include him.