How did missouri's application as a slave state in 1819 challenge (go against what was already established) u. s. law?

Basically, US law established that states north of the Ohio river would be slave free states. Since Missouri is north of the river according to most of its territory, it would have to be a free state. They didn't want this and wanted slaves so the Missouri compromise had to be made in which it was explained under what conditions would Missouri be allowed to enter as a slave state.