In the early 1800s, at the time the Missouri Territory requested statehood,

Slavery was still an issue, so Congress had to pass the Missouri Compromise. There were equal amounts of slave and free states before Missouri requested statehood, so there was much debate on whether Missouri should represent a free or a slave state in Congress, meaning whichever group it represented would get more representation in Congress. So, Maine was admitted as a free state at the same time Missouri was admitted as a slave state, creating the Missouri Compromise.