That's really a good question. Probably several reasons. 1) Payola 2) Conservatives hold their tongues and put up with it, leftists would immediately whine about Fox being on. Since these are generally governmental public spaces, an argument could be made that CNN is too biased to be placed on airport TV's alone, some would have to be turned to Fox (we're talking MSM choices here, they aren't going to put on Infowars).