As is the rest of the media. I've followed politics since 1980. No single candidate has ever had this disproportionate level of coverage. I don't have the numbers, but it wouldn't surprise me if he has received more press time than all of the other candidates in both parties combined. CNN has been particularly egregious, making sure almost 30% or more of its daily programming is dedicated to giving Trump a platform.
Why? I can't really say. Ulterior motives are always difficult since people typically don't admit to them. But you could look more foolish than suggesting that the media is doing this on purpose because it believes Trump is their only hope to keep liberalism alive in the White House. After all, despite the media's best efforts, most Americans know our country is in dire straights. Most know that the international scene is looking bleak, and apart from diehard partisans, it isn't all Bush's fault. Most non-liberals, even if they aren't conservative, are alarmed at the erosion of liberties and assaults on freedom from the federal level. So despite the media's 'everything is awesome!' narrative, a Democrat is going to have an uphill battle against almost any credible candidate. Especially when choice the Democrats have given is between a 74 year old culture-of-death Socialist Santa Clause and a corruption laden candidate with more baggage than a Disney Cruise liner.
So yeah. Just like the media elevated Romney in 2012, it looks a lot like it's doing its best for Trump. The reasons might be different than I think, but I have a feeling I'm not too far off the mark. And neither is Rubio. And those who invoke the Murray Gell-Mann Amnesia defense and declare the media spot on neutral and perfect about everything except what they value, I'm afraid it's not likely to answer fact of the media's coverage improprieties any better.