"Biggest reason:" He wasn't Hillary or some career politician who's done little, or nothing, for the middle class and inner cities, despite talking a big game. Hillary had decades of incompetence and corruption/lies going into the election, not to mention a smug air of entitlement. The political elite are completely out of touch with every day Americans, it's a modern day aristocracy. Her open borders nonsense isn't something a large enough % wanted either Europe's disaster playing out every day on the news and wage depressing illegals being a fine example. The left has also become race/sex baiting central, also leading the charge on political correctness, or shall we shall censorship. People are sick of it!
Trump may have had no experience in politics, but he a turned million dollars into a fortune, people hope he can do the same for the rest of the country, which was the aspect of his platform people were drawn to.
It's not difficult to see why so many voted for Trump, or turned away from the left. It's probably not going to end well, but it never was going to with Hillary.