Bethesda made Fallout 3 and Fallout 4 because they own the franchise now. Bethesda went to Obsidian and wanted them to develop a spin-off Fallout game that used much of the same resources and engine of Fallout 3, which seemed appropriate enough because Obsidian is made up of some of the lead folks that were at Black Isles Studios who developed Fallout 2.
No one knows if Bethesda would want another spin-off developed by Obsidian or if it was just a one time thing, or if Obsidian would be interested in doing it either. As far as we're concerned, Fallout's future is with Bethesda, simply because they own the franchise.
Personally, I too would love to see Obsidian develop another Fallout game. Fallout 2 and New Vegas are among my favorite games in the series.