The Dems generally don't bully their constituents, so they know they have to throw constituents a bone at least once in a great while. Republicans are all about manipulation and bullying, and it sadly works on enough of the population to get us where we are.

Neither are "left," however (as another responder pointed out to you). I expect eventually Progressives will splinter off into their own party once they've captured enough power, and grown to the level of popularity they should have - they are "left."