It's mainly because of the engine, but in FO3 they kinda overdid the 50:s references, and butchered most of the lore, localizing the FEV virus to the east coast whereas it was supposed to localized exclusively at the west coast, and of course fallout is a future-wild-west-with-some-50:s-references themed game. They also changed the super-mutants into "Steroid-junkie Orcs".
Technically speaking it is not Fallout 3, but it is more of "Oooh, lookit mah I'z oblivion in pozt-apoc setting, and I haz same outdated engin az my predecessor, but you can only chooz to b wun rejs, also, i'z not compatibul wiz oblivion modz, yes, i'm zat much of an asshole"