Most of the places where slavery has been practiced around the world and throughout history have not been hospitable to profitable plantation cotton growing. That hasn't stopped people from owning, buying, selling or being slaves.
If slavery could be legally established in Kansas or Nebraska this would make slavery more secure in its core area further South. Washington and other Founders hoped that slavery would eventually disappear. Free Soilers were following in the tradition of the Founders by restricting slavery and expanding the realm of freedom.
The economies of those places was even worse than the economies of the South. They worked slaves at much lower profit than did the plantation owners in the South, and if they had had their choice, they would have used them to make the sort of lucrative profits that Cotton was making in the mid 19th century. But they didn't have that choice, so they used them in whatever manner seemed to them most profitable.
If slavery could be legally established in Kansas or Nebraska this would make slavery more secure in its core area further South. \
More secure than impossible to abolish? How do you get more secure than it already was? No, it wasn't about the security of slavery, it was about the destination of the profits of slavery. It was about congressional representation in Washington, and I have come to doubt any other claimed explanation. It is no coincidence that millions of dollars were riding on the make up of the congressional delegation from territories that became states, and in every case those millions represented money that would fall out of the New York/Washington DC economy, and be spent in the various economies of the South.
Sure, there were the liberal nut-jobs then as there are now, and in those day the liberal cause of the day was abolition. Nowadays it's transgenders and abortion. But the liberal nut-jobs hardly made up a majority of the people back then as now.
The rest of the people were astro-turfed into believing that there was this great worry over the "expansion of slavery" and the "Free Soil" party was there to stir the pot. "free soil, free speech, free labor, and free men." They were literally trying to bribe people with offers of land to get them to move to Kansas and vote against slavery. Why? Because 200 million per year (in 1860 dollars) hinged on Kansas not aligning with the Slave states in their congressional representation.
Of course nobody paints a movement as being for the purpose of profit, they color it as a just and noble cause. This is why the Liberals today say "REPRODUCTIVE RIGHTS!" when they talk about the lucrative industry of Abortion.
But the dollars tell a different story.