Takadanobabaalien

To an extent, the choices we make by voting eventually become out of our hands anyway, so there's a feeling of "it won't matter if I do or don't." Another big thing to remember is that the president is largely a figurehead when it comes to making policies and international relations, and Congress is what makes the real changes. Unfortunately, running for Congress isn't glamorous, can't really be made into a media circus, and no one gives a shit about it, so that only further amplifies the feeling of "It won't matter if I do or don't."

Other people feel neither main political party represents their best interests, so not voting is a sign of protest. There are independents who run for president, but as shown in this election and in the 2000 one, they never get far and they tend to just detract votes from one candidate, so said votes are essentially a waste.

Lastly, a lot of jobs (especially service jobs and blue-collar ones) don't close for election day. If you have to work and can't take time off to do so, that means you can't vote until either late at night (which might be hard if you have a family) or not at all. I was fortunate that my office closed in the morning for that very reason, but I know there are a lot of people who are not in the same boat. Some states do allow for "early voting" now, which means you can go vote a few days before. There aren't as many locations to go do this though. Boston, for example, has a few dozen polling locations  that you can be assigned to based on where you live, but for early voting, only one of them is open per day for the entire city. That usually means huge lines and it will again eat up a lot of time you might not have if you're working.

^^

What they said.

So, guys, let's make America great again