The issue was states rights. slavery is not discussed in the constitution until the 13th amendement. since it wasn't in the constitution it was a decision that belonged to the states. if the balance of slave and free states isn't kept then the government was showing if favored one side over the other when they didn't officially have a position on the subject.
Improved transportation greatly affected farming since it allowed vital resources to be quickly shipped to the farmer, and for the farmer to export his goods far more easily to locations further away.
