What helped Oblivion's case was also that you started the game as a nobody, and that the storylines were actually good for the most part. Thieves Guild and Dark Brotherhood were the highlights of base Oblivion, along with the Shivering Isles expansion. 3 of the best storylines that game had, memorable to this day.
You stole an Elder Scroll, you killed everybody in the Dark Brotherhood by yourself, and you ended up becoming the new Sheogorath, all while being just a random mortal.
How the hell can you beat that?