To expand on this slightly: those further left not being a large enough bloc to exert control of the party is entirely why the Democrats are so mixed in the first place; the Democrats are still very much a coalition party and while that doesn't mean that various interests in the coalition are nessecarily opposed to each other it does mean that they have very different priorities that results in a party that doesn't, say, have a clear loud message on immigration because immigration is only a huge make or break issue for some fraction of the base. This is not how the GOP base is structured whatsoever and it always places us at a disadvantage.
To be clear, I'm not happy with this state of affairs, nor am I defending it as the best way for the party to be, but I do think that this is an issue that is less about incompetance from politicians and more about a very scattered voter base that usually bands together out of survival. Unifying the voters is the most effective way to fix this