If not Donald, someone else would be Trump. America has been waiting for him.
Trump Is...
Trump is a racist, who feeds back to an angry America what it wants to believe -- that its problems are the cause of Blacks, Latinos, women, and immigrants. Trump's supporters want to think those groups rose up to take their jobs, but instead do not realize they themselves were just pushed down into the same ranks America's traditional oppressed occupy. Trump tells his supporters they are victims of an unfair world, ironically true in a way that must make him snigger off stage.
Trump is a fascist, who promotes an America at war with anyone who challenges its self-image. For the past 15 years, politicians and media have fed the raw meat of revenge to Americans, so it should be no surprise that Trump's shouts to invade and bomb and torture are so enthusiastically received.
Trump is an opportunist, knowing that social issues such as gun control bring in the crowds (no one is going to repeal the Second Amendment) while mattering not a whit to the world he and the other power brokers care about. What is really important is maintaining the military industrial complex and being able to manipulate the tax/economic/investment systems. In their minds, only stupid poor people worry about transgender toilets and the like. We don't need bread and circuses, we have Fox.
Welcome to Weimar: America is Now Ready
Others have tried to be Trump and failed. Nixon had most of the pieces in place but fell victim to a media that still cared back then to do its job. That's no longer a problem.
Reagan came very close, but had America's reliable Russian enemy pulled out from under him. The various evangelistic candidates of the '80s and '90s also tried, but most fell victim to sex and money scandals.
In 2016, America was ready for its little Hitler.
The Bush years weakened the institutions of America (the press in particular.)
The Republican party, by pushing forward Caligula's horse in the form of Sarah Palin, laid bare its contempt for the electorate. The electorate ate it up as great TV.
Obama, full of pretty words, cynically picked up his office and that Nobel while twisting the knife with drones.
The Congress threw away its Constitutional role and empowered a strong man executive because it could not agree on anything else.
The one percent nailed things after the 2008 economic crisis (Throw them in jail? They got the government to pay off their financial malfeasance for them), pretty much ensuring their hold forever on power, even as we down here argue over what level of poverty a minimum wage should allow us.
A Man of His Time
Trump sensed all this perhaps without even understanding it, like a predator catches a scent. He added in his own amazing media skills. He realized he could say anything, and contradict himself from speech to speech. The media and public certainly wouldn't care, they'd encourage it as entertainment.
All that said, no one should count Hillary out; she is armed with her own skills. She indeed may end up as president. But that only means the next iteration of Trump, whoever he or she is, waits a bit longer. The broader processes of history can be delayed but not denied.