Because Senate was already 50% free states, 50% slave states, (11free/11slave) and adding Missouri as either a slave or free state would offset that balance and give more power to one side than to the other until Maine decided to want to be added as a free state, then Missouri came in as a slave state and therefore they wanted to keep the number even so the higher one wouldn't take over the other.
There was and even amount of slave and slave-free states
admitting Maine to balance the slave issue ( answer for A + )
The Missouri Compromise temporarily settled the debate over slavery by allowing Missouri enter the Union as a slave state. Maine was allowed to enter the Union as a free state.
The Missouri Compromise temporarily settled the debate over slavery by allowing Missouri enter the Union as a slave state. Maine was allowed to enter the Union as a free state.
The Missouri Compromise temporarily settled the debate over slavery by allowing Missouri enter the Union as a slave state. Maine was allowed to enter the Union as a free state.
Missouri would be admitted to the union as a slave state and Maine as a free state.
The decision of whether to admit the new South Western territories to the Union as slave-states or free soil. The Compromise allowed no new slave-states North of the parallel that marked Missouri's Southern border.
The Missouri Compromise of 1820 allowed Maine to be admitted to the Union as a slave state. This was to counter balance having Missouri enter the Union as a slave state. The Missouri Compromise of 1820 allowed Maine to enter the US as a free state and allow Missouri to enter as a slave state.
Missouri was admitted as a slave state. This was part of the Missouri compromise in which Missouri would enter as a slave state and Maine as a free state.
Missouri just joined the union as a slave state
Missouri
Missouri's application for statehood in 1819 raised the issue of whether slavery should be allowed in the new state, leading to fierce debates in Congress about the balance of power between free and slave states. The Missouri Compromise of 1820 emerged as a temporary resolution, allowing Missouri to enter the Union as a slave state while Maine entered as a free state, and prohibiting slavery in the unorganized territories north of the 36°30’ parallel.