It wouldn't be "Fallout" if it took place before the nukes and second it would just be like 1950's GTA.
I would have to agree there.
Fallout is a post apocalyptic role-playing game. In other words, it's an RPG in a post apocalyptic setting, while obviously, there's quite a bit more background to the Fallout setting than "It's the future and there was a nuclear war." the post apocalyptic setting is one of the defining traits that makes Fallout what it is. A game set before the war would lack what makes Fallout, well, Fallout. I would not want such a game because it fails to capture the most basic thing that defines Fallout as what it is. A prewar Fallout would be kind of like making a Star Wars movie without a war in it.
Now, I did not object so much to how Operation Anchorage handled it, while as far as design and gameplay goes, that particular DLC may not have been what I would consider an ideal addon for Fallout 3 because it lacked the free roaming and role-playing aspects and instead tried to be a military shooter in a game that really doesn't do as good a job at being a first person shooter as a pure FPS should (Note that this is not meant as a criticism, combat is not the only thing Fallout offers, so it doesn't necessarily need to be as well designed as in a game where it's the main focus for the game to hold up.) it did give us a look at an aspect of Fallout's prewar America through a relatively short simulation, rather than giving us a full game in a prewar setting. In this case, the rest of the game was still post apocalyptic, we just got to see a bit of the prewar world in the duration of the quest, that was sort of alright I guess, mostly though, Fallout games should focus on the world after the war, it's not like details like who launched the nukes first or why matters when everyone has been hit just as bad anyway, so we don't need to know every single detail of the war.
As for a game set during the war, as has been said, I don't think a two hour long nuclear war would offer many interesting oportunities for an RPG.