In basic terms, the north states didn't want slaves and the south states did. The boundary line deciding if a state was north or south, was the Ohio river. Anything below that automatically became a south state. The problem was that the House of Representatives is based off of the population of a state, so if Missouri became a state, the balance between north and south representatives would be thrown off. The Missouri compromise cut the boundary line straight down the middle, so that the north side of Missouri would have north representatives and visa versa.
