A good explanation

Mark Steyn went to a Trump rally in Vermont. As usual, his synopsis is spot-on. You should absolutely read the whole thing, but this passage is crucial:

~MESSAGE DISCIPLINE: In fairness, he is (or was) actually competing against Pataki, and still is (just about) against Rand Paul. But he also did a couple of minutes on Martin O'Malley. He'd been talking about the crowds he's been getting, and he'd said that when he goes back home his wife asks him how the speech went and whether anyone was there. Because the cameras stay directly focused on him and never show the audience. And he thought at first this was because they were fixed and hammered into place - until a protester starts yelling and then suddenly the cameras are twisting around like pretzels, no matter what corner of the room they're in. Anyway, at some point, he mused on a Martin O'Malley rally at which apparently only one person showed up. So O'Malley talked with him one on one for an hour, and at the end a reporter asked the guy whether he would be supporting O'Malley. And the fellow said no.

And we all laughed, as did Trump.

Now, short of the mullahs nuking Hillary in Chappaqua and the following day Kim Jong-Un nuking Bernie in Burlington, there is no conceivable scenario in which Trump will be facing off against Martin O'Malley. So talking about him is a complete waste of time - and Karl Rove says that campaigning is all about the efficient use of the dwindling amount of time you have this close to Iowa and New Hampshire. So doing ten minutes of knee-slappers on Martin O'Malley is ten minutes you could have used to talk about Social Security reform that you'll never get back.

Maybe Rove is right. But as a practical matter it's led to the stilted robotic artificiality of the eternally on-message candidate - which is one of the things that normal people hate about politics. And Trump's messages are so clear that he doesn't have to "stay on" them. People get them instantly: On Thursday he did a little bit of audience participation. "Who's going to pay for the wall?" And everyone yelled back, "Mexico!" He may appear to be totally undisciplined, yet everyone's got the message. Likewise, his line on an end to Muslim immigration "until we can figure out what the hell's going on" is actually a subtle and very artfully poised way of putting it that generates huge applause. Trump has such a natural talent for "message" that it frees up plenty of time to do ten minutes of Martin O'Malley shtick.

Emphasis mine. In contrast, consider this criticism of Trump from a bien pensant at Salon:

The point is that when a large portion of the electorate is fooled into adopting an ideology of ignorance, spaces open up for charlatans, frauds and demagogues to gain political power over the masses. This is precisely what we’ve seen with the rise of Trump in 2015, and it’s why he’s a symptom of the disease rather than a cause, as mentioned above. Indeed, according to a recent poll, Trump’s support base consists of the least educated Americans. As a Washington Post article put it, “Even when pollsters took race out of the equation — to the extent that that is possible since the Republican base is overwhelmingly white — and looked at all Republicans, the relationship between education and Trump support was pretty clear.”

This conclusion is consistent with a Boston Globe analysis that found that Trump talks at a fourth-grade level. And it fits with the observation that the more outrageous, harebrained, foolish, and asinine Trump’s claims — from John McCain not being a “war hero” to shutting down parts of the Internet to violating the Geneva Convention by killing terrorists’ families to temporarily keeping Muslims out of America — the more Trump’s poll numbers seem to rise. Holding beliefs that are properly tethered to reality via the best available evidence doesn’t matter much to the army of quasi-fascist conservatives who’ve been hypnotized by Trump’s shallow charisma.

Again, emphasis mine. The sneer about "talks at a fourth-grade level" demonstrates that the author of the piece, Phil Torres, is more committed to a world view than he is to understanding what's happening in the campaign. Trump is effective precisely because, as Steyn notes, his messages are easy to comprehend. Anyone who writes for a living should understand that reaching the largest possible audience is a good thing, especially if you are asking for someone's allegiance. Trump understands that the vote of someone who understands the world at a fourth-grade level has equal value to the vote of a Wharton grad. Perhaps more, if the voter with the fourth-grade understanding lives in an early primary state.

Note also that Trump was speaking in Vermont, a left enclave that is the home of Bernie Sanders. Back to Steyn:

When was the last time a GOP presidential candidate held (in the frantic run-up to Iowa and New Hampshire) an event in Vermont? Every fourth January, Republican campaigns are focused on the first caucus and the first primary states, as Bush, Rubio, Christie, Kasich, Huckabee, Fiorina et al are right now. But in fact the Green Mountain primary is on March 1st, and its delegates count as much as any other state's. In recent cycles, the American electoral system has diminished and degraded itself by retreating into turnout-model reductionism and seriously competing only over a handful of purple states. Even if he's only doing it as a massive head-fake, Trump understands the importance of symbolism: By going into Berniestan, he's saying he's going for every voter and he's happy to play down the other guy's half of the field.

No one has tried to be a national candidate on the Republican side since Ronald Reagan. Trump is not Reagan, yet it's important to remember that Reagan was known as The Great Communicator because of his ability to reach audiences where they were. Trump has the same skill set. He's going to be very difficult to beat.