The Missouri Compromise was trying to keep the power equal between the North and the South. The country was already extremely divided and neither side wanted a state added which would make the power uneven between the states. In order for Missouri to be admitted as a slave state to make the South happy, then the North also wanted a state added so that the South wouldn't have more states than them. And vice versa. The South didn't want Missouri to be admitted as a free state to give the North more power. So Missouri was admitted as a slave state and Maine was made as a free state to try to keep the balance between the two sections more equal.
