The loss of factory and industry workers.
Step-by-step explanation:
During this time, the North was very industrialized and had many factory jobs. This was very different compared to the South, as the Southern states focused on agriculture and had little to no industrialization present.
If there was going to be cheap land sold in the more agricultural part of the United States, that meant that many people would take this offer and live humbly.
The North did not want this, they needed a lot of workers to keep things up and keep the industries growing rapidly, they could not afford to lose precious workers that were helping the North grow, that's why they were very much opposed to the cheap land that was being sold in the West.