Erika Fry collects a substantial amount of evidence that political journalists are doing their damndest to keep the GOP primary going. Ed Kilgore thinks that's not the only bias at work in election coverage:

[H]orse-race enthusiasts are not the only poltical journalists with corrosive biases: others, particularly in Beltwayland, are smug purveyors of various Iron Laws of politics in which electoral outcomes are dictated by “insiders”—i.e., their sources—months or years in advance. So sometimes campaign coverage seems to be divided between those who breathlessly over-react to every twist and turn in polling numbers, and those who think none of it’s worth covering at all. Indeed, you sometimes see both tendencies in the same publication.