While getting all of that land in the Louisiana Purchase was cool, and the bargain of the century, there was little thought given to how the population might expand into the new territory, and whether or not slavery would expand with them.
The North, while not abolitionist, was not excited about the slaveholding South getting more power in Congress, so they wanted assurances that the balance of power would be maintained. Henry Clay stepped up and negotiated the Missouri Compromise which allowed Missouri in as a slave state and Maine as a free state at the same time. They also came up with a method to determine whether new states would be slave or free by establishing the 36 degree 30' N latitude line as the dividing line between future slave states and free states.
The Missouri Compromise worked for a while, but it just showed America how difficult the slavery issue was getting for the country as it expanded west.