After running through the obvious causes, Rick Hertzberg identifies another one -- social discomfort:
There may also be a management problem, a problem of staff morale. For a news professional, even one of conservative inclinations, Fox is a problematic place to work. In addition to suffering the humiliation of having to slant one’s reporting to serve the interests of an increasingly unhinged Republican Party, Fox employees cannot help but be aware of the contempt and/or pity, tacit and/or open, of their peers. I suspect that this problem is at least as severe at the peon level—bookers, interns, researchers, receptionists, coffee fetchers, mailroom guys, etc.—as it is for correspondents and producers. Just getting rid of Beck won’t remove the air quotes from around “Fox News,” of course. But for members of the company rank and file, Beck’s departure could mean a slight reduction in the percentage of people encountered in the course of a day’s work who, upon hearing “I’m with Fox News,” physically recoil.
I would guess that played an important role.