From the Mark Steyn article:
The media blackout of 1998 [when McCain told a tasteless joke involving Chelsea Clinton] is in striking contrast to the wall-to-wall coverage of Trump's "gaffe". Two decades back, the media loved McCain because he was their kind of Republican: that's to say, he spent more time attacking other Republicans than he ever did Democrats. And they wanted him to keep on doing that, so they closed ranks around him. Whereas no one wants what Trump wants to talk about to be part of the 2016 election conversation, so anything must be seized on to disqualify him from participation. And, if nothing else is to hand, a sneer at John McCain will have to do.