The north states did not want Missouri to be a slave state because its in the North and they did not want the South to think they are overpowering because there are 11 non slave states and 12 slave states .
This was a compromise that make sure that slavery wouldn't become the deciding factor in westward expansion and allow the nation to move on politically.
The south wanted Missouri to be a slave state because they wanted all states to be slaves and have more people to trade slaves with.