Zenimax Media/Bethesda

 
That's sad because Bethesda must be mad that FO:NV was better than FO3, and is highly regarded as one of the best FO games of all time.
You never know why these things fall through. Maybe Obsidian wasn't willing to sign another deal that hinged on a 90+ Metacritic score. Alternatively, maybe at the time Bethesda was focusing their efforts on killing Human Head Studios.