Here's a great reason why teachers in South Dakota need to be paid a lot more: They have to buy classroom supplies with their own money. Most K-5 teachers have to buy their own supplies for their class because the school doesn't supply them. Most of the teachers are forced to spend 50% (in some cases I've heard it as high as 80%) of their income on those supplies.
So SD teachers are already getting paid very little, but they are also forced to spend that little income on their class. Some schools might help the teachers out, but they have no money to spare.
So... why aren't we paying teachers adequately?