Just because they own all the pieces (the Fallout franchise, the engine Bethesda created for F:NV, Obsidian Entertainment) doesn't mean there's a workable concept for the game.

Obsidian put out two games in the past two-ish years: Grounded (a co-op survival shooter) and Pentiment (a point-and-click murder mystery set in the middle ages). Both were well-received. In addition, Obsidian has also announced The Outer Worlds 2, a sequel to a game that (fairly or unfairly) was compared to F:NV.