During the 1850s Southern households were constantly rolling west in search of new land. Letters from new settlers in east Texas describe the land as three times as productive and much cheaper than comporable land east of the Mississippi - not scientific by any means, but you don't usually question such claims made by a familiy member. The main goal for many lesser slaveholders (fewer than twenty slaves) was to make money, and make it quickly. The right way to do so was to invest in land and slaves, and move west.

By typical planting methods, it took about two decades for cotton culture to wear out the soil. Since the invention of the cotton gin, most Southern men expected to move when fresh land opened up somewhere else. That's why the issue of slavery in the territories split the Democratic Party in 1860. Citizens of the slave states didn't particularly appreciate being unwelcome in any part of the country.

If they had read Lincoln's speeches in 1859 more carefully, it's evident from his language that Northerners did not want the presence of black people in the western lands. IMHO much of the opposition to slavery was based on what we would call outright racism today. I have quoted Lincoln's remarks recently in which he mentions (slave n-word) as undesirable to white labor in the west.