In an interview with The Daily Beast's Howard Kurtz, Fox News chairman Roger Ailes revealed that he has recommended to Glenn Beck that he not attack Republicans:

What about Beck's other inflammatory outbursts, such as calling Obama a racist? Ailes says that everyone who ad libs for a living makes mistakes. But admits to asking Beck to watch his tone: “He and I have had conversations and lunches where I say, 'What the hell are you doing, man?'...Beck trashes Republicans every night. I've said to him, 'Where the hell are you going to get your audience if you keep this up? You're trashing everyone.'”

Beck, of course, spent the weeks before the 2010 election imploring his audience to vote for Republicans, while praising GOP candidates and savaging Democratic ones. Beck turned one pre-election show into an hour-long telethon and get-out-the-vote drive for three Republicans he hosted, at one point turning to Sen. Jim DeMint and asking, “What kind of help do you need?”

As for Ailes pointing out that criticism of Republicans could lead to Fox News' audience going elsewhere for their news, that's as good an explanation as any for the network's coverage of the Bush administration.