This statement shows just how much you have accepted the narrative you've been taught. I actually went to the trouble of looking at this "spread of slavery" and what i've discovered is it's a lie. Slavery could not spread to any meaningful degree into the territories because plantation style farming was impossible in the territories at the time.
But we've all been taught the lie, and so now people simply repeat that claim with no knowledge regarding what is actually true.
The cotton shown in New Mexico, Arizona and Southern California can only be grown in modern times due to massive irrigation systems which could not have been created in the 19th century.
The truth is, that there were less than a dozen slaves in all the territories up until 1865. There was no demand for slaves in the territories. There would have been no "spread" of slavery.