It is all about politics. Politics, at least in the US, has degenerated to the point of thuggery. Each major party foments as much hate as possible against their opponents because neither wants to be fairly scrutinized. At the very least the parties have few, if any, ideas to help their constituents.
Here is what I call my "tin foil hat theory": The single largest revenue stream in the world is the US federal budget. Trillions of dollars per year flow through the Treasury, most of that ultimately going to civilian vendors through a contract process. Any criminal organization would dearly love to get a piece, even a tiny one, of that action. The hard part is that it is administered by a huge organization and millions of low level bureaucrats. The easy part is they are ultimately products of a political process: Elected congressmen pick the top level of that; from there on down it just grows. The easiest part is it all comes together at a pair of points: The Republican and Democratic parties. It takes shocking little seed money - probably less than a billion dollars - to gain influence in each party leadership over a period of several years. From that point the money just rolls in, using the New York model of "every businessman has a 'guy'." Contracts are finagled - nobody really believes there is adequate oversight of the immense US contract process - and the money rolls in.
I have occasionally brought up my tin foil hat theory with friends and acquaintances, just for fun. The surprising thing is the most common response is "What makes you think that has not already happened?" 100% of New Yorkers have responded that way.
To the original question: Trump is an outsider, even within his own party. Ronald Reagan was the previous record holder for an outsider President, and at least he had held office. Trump, more than anything else, is a symbol within both major parties of a serious loss of central control. Maintaining public discontent, no matter how pointlessly rabid, is critical to both parties. In a culture that has embraced TV shows that focus on what would be evidence of personality disorders in the real world it is not hard to emulate George Orwell's vision of 1984 with the Two Minutes Hate.