1) There are far too many people willing to have an obnoxious inarticulate inexperienced sleazy narcissist loose-cannon TV personality as President of the United States of America.
2) Our voting system is absolute bullshit forcing what should be a false dilemma on voters and raising it up as a choice by the voters, when really it is the voters being forced to place a single vote “for” one of only two candidates widely proclaimed as the only two electable choices – each one selected by one of the two mainstream political parties which dominate US politics, both of which are deep in the pockets of manipulation by the mega-wealthy, giant corporations and “special interests”.
3) The other “electable” choice offered by our sorry excuse for a voting process, was Hillary Clinton, who ran a campaign that stank of the establishment and often felt like she thought it was a foregone conclusion she should get to be president, with her supporters even including the phrase “it’s her turn”.
4) Clinton also has a long-standing movement of people who have hated her for decades.
5) Clinton gained many more people who hated, resented, and/or distrusted her during the primary campaign, especially in her race against Bernie Sanders, where despite Sanders miraculously giving her a surprise challenge despite starting with little name recognition and all individual non-corporate donors and undermining by the Democratic Party, the Democrats tries to deny that and blow off that Sanders had actual authentic support and enthusiasm. It also was clear at many points during the Primary that Sanders was being denied a real chance at the nomination, again the main reason being that the DNC had already pre-selected her. Many people resented that and she lost many votes from it.
6) Both “electable” candidates had (and continue to have) negative approval ratings. That led to low voter turnout. Many people who would prefer Clinton to Trump did not vote, or did not vote for her, because it was assumed Clinton would win, and/or they wanted the DNC to notice how pissed they were about the primary and/or Clinton’s lazy entitled campaign. But Trump fans were fans and did tend to vote.
7) Also, when voter turnout is low, the proportion of wealthier and statistically more conservative voters is higher, and vice versa. Republicans tend to win low turnout votes, while Democrats tend to win high-turnout votes.
8) Also, US politics have become very polarized and many Americans vote on party lines. Trump’s terrible behavior increases that effect. Many people voted Trump not for Trump himself but because he was the Republican candidate, because he was not the Democratic candidate, and because of other party-line issues such as abortion or immigration.
9) So how did Trump become one of the two choices? Well, the other Republican candidates were all terrible and boring and reeked of the same old establishment politics more Americans are sick of, and more people preferred Trump to them because he was not an establishment politician and was telling those Republicans off. Trump was also just more noticeable and attention-getting than the other Republican candidates.
10) Clinton was also a woman. Sexism is still a factor in US politics.
11) The new media normalized Trump more than he deserved to be normalized as a candidate.
12) Some people had other specific reasons not to vote for Clinton, such as seeing her as intent on getting us into wars, and/or trade treaties that would give away many sovereign rights to corporations.
13) Before the election, Trump also had some people hoping he might be a random non-establishment chaos factor in government. Many people feel a lot of resentment for establishment big-party politics and business as usual, and Trump’s randomness and “drain the swamp” line had some people (even me) somewhat hopeful he would cause problems for corrupt business as usual. (Unfortunately, that turned out not to be the case in practice, as he turned out to be utterly in the pocket of corporate interests and to be an experiment in throwing out all real pretense of being anything but a tool to give as much away to corporations as possible).
14) There are also a lot of racists and xenophobes and gun nuts and idiot Bible thumpers in the USA, and they all saw a champion in Trump.
15) Trump also champions the “anti-intellectual” vote, being a great example of how idiocy can prevail.
16) There may also have been some electronic voting machine issues – since they don’t leave a real paper trail, we’d likely never know if there were.
etc.