They get so much grassroots support because...
This is your answer. The change we want isn't going to happen at the federal level only. The DNC has had a really terrible leader up until recently, but even so, they don't have a good pulse on the left-electorate. They can't seem to figure out that the country isn't center anymore.
The reason we didn't have a "red tsunami" at the midterms, which going on historical trends should have happened, is because of the extremely hard work of grassroots orgs that mobilized the voters on the left (typically low-turnout voting bloc) in local, state, and federal races. And they did all that on a shoestring budget, with little to no help from the DNC.
The change is going to have to be from the local level up. That's how the GOP got to their "minority rule" status, and it's how we'll win back true democracy.