In Trump's case, that's pretty true. The media didn't bother (during the primary), and none of the other 16 in the clown car bothered to do or release any oppo research. These days the media will use the fact that his opponents didn't to justify them not doing it. Cover the controversy is normal, just covering seems to be in itself controversial. None of this is meant to criticize good journalists doing good work, just the weird culture where "examining a candidate's past" or "fact checking" are akin to taking sides. The good journalists seem to actually have to argue their case or be partisan! Of course this view only applies sometimes to some candidates. Sometimes the opponents "raise questions" and sometimes it's enough that the voices in your head do. I'll never quite figure it out.
As for Trump, well, as I've said, I probably didn't think much of his candidacy when he first announced. I honestly don't remember! Maybe I would have laughed at Keith Ellison early on, too, for suggesting we should take candidate Trump seriously. Maybe not! But he went to the top of the polls pretty quickly and stayed there. When we have consistent polling data, it says something. It was all blinking Trump. In 2012 it was fun to mock the whole thing because every candidate seemed to get their medal for the week, but that never happened this time around. It was all Trump except for that brief moment when it was Ben Carson.