ADS in Doom is a bad idea, primarily because it's been very clearly designed to be fast-paced arena combat where being able to take in the periphery is key to knowing what is attacking you, from where, and where you should go next.
I don't think ADS is a bad thing. But in a game centered around fast movement, what it would probably do is 1) lure you into a false sense of security because you'll likely think you do more damage when ADS, and 2) obscure the periphery enough that you are unable to react effectively to encroaching enemies. I think it would make it harder, not easier, to play.
That said, I wouldn't care if it was there, and who knows maybe they prototyped it and thought it didn't work because of button layout (guns with more than 1 firing mode limit options in terms of where that ADS button goes). But I do think there's a good case to make against ADS here -- my recommendation would be to try and get used to it. Some of this stuff is really just in your head, since it wouldn't provide a mechanical benefit as it would be implemented here (your aim won't get better, you won't see more, you won't do more damage).