Heh. Stated like that, you make it sound as if Bethesda slammed the door in their faces, as though it were personal - like they were being spiteful or something.
Qualified people apply for jobs and get turned down every day, in every industry - game developers and the gaming industry are not an exception. Usually there are a boringly normal number of reasons why this happens.
Hmmm...
I cannot imagine if say... I were working on the next Dune game and [the late] Frank Herbert were alive, and offered to consult... I cannot imagine considering to say "no".
The situation here is different, but its not as though the company can't hire them on their terms. What does come to mind is fear of the "prima donna dev". Had any of them been in on the project, they may have stole the show hands down for simply being there. They may have had a difference of opinion ~Would anyone want an original dev to leave their followup project for creative differences?
My assumption is that they wanted a TES4 clone dressed as Fallout (this is not a slight, this is to me... self evident). That's where their established market is, and that's what their reputation demands. What they made is exactly that, and its beautiful to look at, and does exactly whats was expected of them ~just not IMO what's expected of Fallout.