There was some shady stuff on behalf of the DNC but he legitimately lost. He didn't get the votes. Because his voters didn't vote in the primaries. A number of reforms have been made to the primary system since then, a bunch of the people who oversaw that primary got fired, and many states are now moving towards ranked choice voting which will eliminate the need for primaries entirely. If half the people who complain about how voting is useless actually participated in the primary process, our political landscape would look a lot different. I used to be one of those people, I get it, the whole damned thing is a bit of a racket, but it doesn't change that voting takes 5 minutes and has a concrete impact on who runs the government.

Edit: And that's the presidential race. You can make much more of a difference, and the rules are much less wonky, in local and state elections. Hell, many of those positions are entirely uncontested.