1) He is the most electable candidate on either side right now. Bernie Sanders isn’t nearly as far behind Trump as many observers assume, and he’s more nationally electable than he’s ever been, but he’s a bit too left-eccentric for real viability in national politics.
1a) Trump and Sanders are much closer on policy than mainstream reporters and pundits will allow: e.g., Trump has expressed an interest in abrogating NAFTA and shoring up Social Security, and Sanders has a record of support for lax gun regulations, a longstanding bipartisan position in Vermont.
2) Trump is the least corrupt-looking candidate in the Republican field. His crooked deals with various municipal governments have faded into the distant past, leaving him as the only candidate on either side other than Sanders who is eager to speak plainly about crucial matters having to do with the willful destruction of the middle and lower classes at the behest of sleazy, self-dealing elites. Of his Republican rivals, Rand Paul is the only one who stands out as maybe being reasonably clean and public-spirited. Christie is implicated in the politically motivated fubaring of the George Washington Bridge, which has resulted in criminal indictments against former staffers in his gubernatorial administration. Bush, Rubio, Fiorina, Carson, and Kasich are all flagrantly corrupt, although in different ways, and Cruz is probably only marginally more aboveboard. Excluding Trump and (probably) Paul, the GOP field is beset with dynastic privilege, managerial-class flimflamming, schemes to further the oligarchical looting of the commonweal, unionbusting, multilevel marketing fraud, Curtis LeMay-level international belligerence, and religious lunacy.
3) If Trump doesn’t withdraw from the race or flame out in the primaries, he may well provoke the GOP to destroy itself by way of trying to sandbag his candidacy. The establishment operators are really cross that he’s threatening to piss on their parade, and they’re likely to get desperate. The tide is ebbing around them, and the Donald is pointing out that they’re stark naked.
4) Hillary Clinton is too actively, aggressively crooked to be fit for high office, and a large part of the national electorate, probably a critical mass, distrusts her. Negative opinion of her is almost certainly stronger and more widespread than negative opinion of Trump, especially as more Americans, including longtime disaffected voters, listen to Trump’s arguments. His policies may be inchoate and vague, but they’re articulate and philosophically coherent. Hillary, by contrast, comes across as either ulterior or manifestly without substance.
5) Clinton and Bush (these names again) are both angling for a coronation, and they’re both butthurt that they have to compete for the nomination just like anyone else. The American public has no appetite for this whiny entitlement on the part of obscenely wealthy dynastic heirs who live like royalty. Trump lives in obscene wealth, but he’s more open about it than any of the wealthy career politicians running against him in either party, and he obviously doesn’t consider it beneath himself to have to compete in a crowded, chaotic, treacherous field. He’s cagey about the family money and connections that he inherited, but he’s less cagey about his socioeconomic circumstances than his rivals, except (maybe) for Sanders and Paul.
In the land of dog’s asses, the horse’s ass is king. These are mysteries only if you’re super sheltered from the riffraff. I’ve been downwardly mobile for close to a decade, so I’m not out to lunch with the chattering classes or the highbrow racketmen. There are reasons why Trump resonates with normal people, and I can assure you that it is not just because we’re a bunch of bigots.