1) Fox News loves a good on-air rumble. NPR is Lawrence Welk for baby-boomers.
2) Fox News thinks that in the US, lefties, while few in number, punch far above their weight culturally. Lefties agree.
3) Fox News think that the Democratic Party's form of an expanded social services net & interference in the corporate world is but a prelude to socialism. Lefties hope that's the case.
NPR, on the other hand, is basically the urban white wing of the Democratic Party in front of a microphone. It really doesn't want to give Lefties a platform to ask liberals embarrassing questions (like, "Gosh, just how long does it take to close Gitmo?") or to spout off with some Lefty hate-speech (e.g. New Black Panther Party), which might remind NPR listeners that their side is no where near as rational & nice as they think they are.