I'm not quite sure how to answer this question because it is worded horribly
The answer would be the availability of new farming land in the west, while back east, most of their land had already been taken.
Well, it decreased the demand of Slave Labor.
Back then, the technology advancement , especially in agricultural sector, significantly increased the total production. This cause the demand of overall labor decreased. The northern US, who utilized more of this advancement more than their southern neighbor, manage to produce hundred times more output than the southern who still depended on their slave labor
