Why does the compromise of 1820 add two states, 1 free and 1 slave state?

In 1820, amid growing sectional tensions over the issue of slavery, the U.S. Congress passed a law that admitted Missouri to the Union as a slave state and Maine as a free state, while banning slavery from the remaining Louisiana Purchase lands located north of the 36º 30’ parallel.

Idon’t know who knows but i know who does not know me