Answer:
Due to Congress not having the authority to do so, slavery could not be prohibited in territories.
Step-by-step explanation:
The Missouri Compromise is an act of Congress (1820) by which Missouri was admitted as a slave state, Maine as a free state, and slavery was prohibited in the Louisiana Purchase north of latitude 36°30'N, except for Missouri.
Brief Overview:
The north objected to Missouri's desire to join the union as a slave state because it would upset the senate's balance. Henry Clay comes up with the Missouri Compromise, which keeps the balance by allowing Maine to remain a free state while allowing Missouri to join as a slave state. However, Clay showed foresight when he added that every new state that joined to the left and to the north of Missouri (within the Louisiana purchase) would be free States. There is a specific set of coordinates for this that I can't for the life of me remember, but I don't think it's very important anyway.