There are leftists trying to gain a foothold in the Democrat party, and some are succeeding (see the latest New York mayoral primary).

Many people ignore primaries, though. You know who does vote in primaries? Older more moderate voters. So progressives tend to lose primaries, then the people who didn’t bother with the primaries complain there aren’t progressive enough candidates in the main elections, so they don’t vote.

If you want more leftist candidates (within the FPTP system), support progressive candidates and show up to the primaries.