Oblivion in particular and Bethesda in general only show the power of good marketing. It also appeared in a favorable scenario for them, so much so, that people don't even mind Bethesda's generally shitty DLCs. Ultima 9 and MM9 came out when crpgs were going through a material and creative crisis in the mainstream eyes, so for them half of the battle was already lost.
Oblivion's success was a freak I don't think is explainable by any single factor.
It was a combination of hype, track history, being flagship title on a new major platform and herd instinct in the context of fending off cognitive dissonance and post-purchase rationalization.
Without this pile-up of factors it probably wouldn't have succeeded.
Then, after this bizarre Rube Goldberg machine lifted it, mods helped keep it up.