It was about the armors and weapons, probably also a lot of clipping issues.Is that so? Super mutant armor works in Fallout 4, and there already are different armor types for various super mutants in New Vegas. Shouldn't be too much of a problem to set armor wearable only by specific races. That stuff was already in Morrowind, where the beast races could not wear boots or closed helmets. They scrapped that with Oblivion, but I think that was less engine limitations and more laziness.
Feargus Urquhart said:"Originally we had this idea where the player would be able to choose between three races: human, ghoul and super mutant," he said. "It was just the engine...
"It really had to do with how all the weapons and armour worked. Trying to have them all work with ghouls and super mutants was just going to be - [Bethesda] felt like it was going to be a nightmare. It wasn't like they said no but it was a very strongly worded, 'We would really suggest that you not try to do that.'"