1st: Interplay abandoned Fallout not Black Isle that would later become Obsidian.
2nd: Bethesda asked Obsidian to make New Vegas
3rd: New Vegas sold very well more so than Fallout 3, and Bethesda/Zenimax is very happy with New Vegas. It made something like 300 million in the first couple months.
4th: Do some research before you spout crap such as this topic
5th: I am sure Obsidian would love to work on another Fallout if Bethesda asked them to do so. Remember its Bethesda/Zenimax's game not Obsidian's.
6th: If you are complaining about bugs, that's Bethesda's fault. They were in charge of the quality control.