Why wouldn't Bethesda let Obsidian make future Fallout games if they own the IP and still get a ton of cash and credit for it? Other than spite.
Actually it's a funny story about that.
Obsidian was given an impossible task of completing a game in 1 year and a half years worth of time. I'm sure the studio was stressed out way more than Bethesda's game studio as Bethesda has an average of 4 years per game.
So I'm sure everyone knows the 85% metacritic bonus bullstuff (if you don't then google it). If the game had 85% on metacritic NV would get an increased bonus (not royalties) the game ended up getting 84%. They didn't get that increased bonus payout and had to cancel some projects because of it
(Score probably would have been higher if Bethesda didn't rush Obsidian on the bug testing, if Bethesda gave them another month to fix some bugs the game would have done better. People say NV is known for it's bugs but that's only because Bethesda rushed them.)
Obsidian asked to help on Fallout 4, Bethesda denied them.
Obsidian states they will no longer do deals with metacritic as it's unfair and completely bullstuff.
After all this crap Obsidian still requests that they would like to make a New Vegas 2. Bethesda still hasn't given them a definitive answer yet.
They care about Fallout so much that they would deal with Bethesda's loving bullstuff just to do it.