Earlier this week Noclip published a documentary about Bethesda that contains at least some of interest to Fallout fans. The documentary is pretty shallow but that's the price of access.
You'll find most of it in this thread. But I'll highlight a few things for you:
1) Morrowind was a surprisingly huge success on consoles and later games were developed with the console market in mind. That's the main cause of the dumbing down. They give examples of how they cut a lot of dialog to be able to fit Oblivion on disc.
2) Development of Oblivion was very long and costly. Hence ZeniMax thought it a good idea to have more than one project going at the same time. Bethesda was asked what they wanted to do, Todd wanted Fallout, and one year later he got it.
3) Fallout 4 wasn't set in Boston originally. At one point they were even thinking San Fransisco, it's mentioned that they asked Obsidian to remove one line in New Vegas about San Fransisco being totally nuked. They wanted the bay area to be an option for future games.