Why was the addition of missouri, as a new state, a controversy under james monroe during his presidency?

I know what is it i think maybe
it is james monroe during his presidency

Slavery became a contentious issue during Monroe’s presidency. The North had banned slavery, but the Southern states still supported it. In 1818, Missouri wanted to join the Union; the North wanted it to be declared a free state while the South wanted it to be a slave state. Finally, an agreement was made allowing Missouri to join the Union as a slave state and Maine to join as a free state. The Missouri Compromise soon followed, outlawing slavery in the Louisiana Territory above the parallel 36°30′ north, excluding the state of Missouri. Although Monroe did not think Congress had the constitutional authority to impose such conditions on Missouri’s admission to the Union, he signed the Missouri Compromise in 1820 in an effort to avoid civil war.