But then you understand that Bethesda, at the time of Oblivion, was already just a rotten zombified body of what the company once was (not that I personally think they ever were anything impressive, but I know Daggerfal and Morrowind have some fans on Codex), without any creative ability to create a new IP. Fallout was a series with a certain reputation, a "cult classic", so obviously they would not only win all the lore to rape in any way they liked, but they would also win even more free press for the game than they already would have on a traditional release. It's a win for them in every way.
The answer is simpler than that. Fallout has great lore, and lore is something The Elder Scrolls has always been praised for. So Bethesda bought the IP to cater to their own fanbase with an open world FPS RPG.
I'm guessing Bethesda is far more troubled over FO76 because they didn't expect their fanbase to hate it, but they did. But when buying Fallout and turning it into Oblivion, they knew very well many fans of Fallout would not take kindly to it, but then again, if they cared they wouldn't have bought the IP to begin with.