Democrats have won more elections than Republicans, yet they have moved to the right. So what will it take to move them left?
They haven’t won enough. If people like Bernie are still losing primaries because “commies won’t win general elections” and Dems still have to go for the “middle-of-the-road” candidate while Republicans can prop up the literal antichrist, that means they still haven’t won enough to cause a shift.
Once they get enough wins (possibly in a row) that Republicans are the ones forced to go for a “middle-of-the-road” candidate, that’s when Dems will actually have to act as a left wing party to get votes.
EDIT: also, unless I miscounted, Dems actually have less wins than Republicans post-FDR.