https://www.kitguru.net/gaming/damien-cox/bethesda-says-its-unlikely-that-another-fallout-new-vegas-deal-will-happen-again/
 
Bethesda has been growing substantially and has absorbed several gaming manufacturers. The feeling is that outside gaming developers are no longer needed at this point in time. Bethesda Game Studios currently seems to work on one major title at a time, with Fallout 76 being the developer’s next game to launch on November 14th, 2018. Both single-player sci-fi adventure Starfield and The Elder Scrolls VI have been announced under the Studio’s banner, but without an exact window of release.

one game at a time is better as the focus is tight and the games will be better 
 

Their loss honestly, but Obsidian did a better job with the IP than they have.

NV was the best by far, quest, roleplaying, actions had consequences, shaping the story ending, crafting, pure volume of quality content.  I had none of the issues most had at launch. The only bad parts a 32bit Frankenstein engine past its prime made to do things it should had not been doing, The environment was not as filled but then again limited resources so something had to give.