To be fair, there were multiple factors.

1. Clinton was a bad choice because the right wing knew all the ways to attack her. They’d been doing so for 30 years.
2. Trump was TRYING to lose (he just wanted to scam the campaign money) but the more reprehensible he got in his attempt to throw the election, the more popular he got with bad people. Had he stuck to just policy talk, he wouldn’t have made it past the primary.
3. The above point made progressives think he was going to lose, so many stayed home on election night. We learned nothing from the re-election of Bush the Lesser after he’d fucked up just about everything he touched. Right wingers will ALWAYS vote in lockstep.
4. Progressives have this weird purity test thing where many won’t vote for someone unless they check ALL the boxes. Clinton was one such imperfect candidate. She didn’t stand for EVERYTHING progressives wanted, so progressives took their ball and went home. Voting is not a cab. It’s a bus. You take the one that gets you closest to where you want to go, and you may have to walk from there.

Thus, we got stuck with 4 years of a guy treating the most important position in the world like a part-time job, defrauding the Secret Service by vacationing exclusively at his properties, and making the USA look like a banana republic with a fake Gucci bag.