this is what a two party system does. if you're going to uphold a two-party system, this is what you're going to get. the democrats are never going to fight on their left because they're never going to be threatened on the left. there has to be another party there.
the reason this is so predictable is because the system necessitates it. in a two-party system, all elections are fought in the center and the nominee of the party is obligated to focus her efforts on winning votes on the other side of the aisle. that's how the system works.
again: don't keep upholding this system, then expect it to do something it's not designed to do. if you want the democrats to swing left, you have absolutely no choice but to create a party on the left for it to compete with.