https://www.pcgamer.com/todd-howard-on-outsourcing-future-fallout-games-i-wouldnt-say-never/

Fallout: New Vegas is the best Fallout in the series. Better than 3, 4 and that isometric carry on from a million years ago. Developed by Obsidian Entertainment, New Vegas may also be the last Fallout game Bethesda outsources for third party development. 

In conversation with The Guardian, company director Todd Howard says Bethesda is in a position now where in-house projects make the most sense going forward.