It’s the structure of our “first past the post” system. Basically, each party gets one representative on the presidential ticket. The two major parties have primaries where the top candidates compete in a vote within themselves, and the winner gets put on the presidential ticket for that party.
The obvious problem with that is that the party convention picks the candidate, not the voters. So it’s possible to buy a party’s candidate or for the conventions to snub popular choice in favor of not shaking things up too much in the status quo.
The latter point, the democratic party picking lukewarm candidates that are moderate at best because the establishment doesn’t want to disturb the status quo, has been a problem for a long time and is a major reason democrat voters don’t go to the polls.