Paul Krugman explains in his blog.
I suppose there are still some people waiting for Trump’s bubble to burst — any day now! But it keeps not happening. And it’s becoming increasingly plausible that he will go all the way. Why?
One answer — probably the most important — is what Greg Sargent has been emphasizing: the majority of Republican voters actually support Trump’s policy positions. After all, he’s just saying outright what mainstream candidates have implied through innuendo; how are voters supposed to know that this isn’t what you do?
I would, however, add a casual observation: at this point Trump has been the front-runner for long enough that it’s very hard to imagine his supporters suddenly losing faith, because it would be too embarrassing.
Bear in mind that embarrassment, and the desire to avoid it, are enormously important sources of motivation. ... Nobody likes looking like a chump, and most people will go to great lengths to convince themselves that they weren’t.
Now think about someone who has been supporting Trump since the summer. For the Trump bubble to burst, many people like that would have to slap their foreheads and say, “Wow, he’s not a serious person! What was I thinking?”
And very few people ever do that sort of thing. ...
... even if Trump does finally decline, his support is likely to flow not to an establishment candidate but to another outsider figure. Everyone who knows Ted Cruz well hates him; in this environment that probably enhances his appeal.
The general election will, of course, be quite different. But it’s getting really hard to see how the GOP establishment reasserts control.
I suppose the answer to the question of GOP establishment control is a brokered convention. But the monster under that bed is a 3rd party run.