Why was there a big controversy when Missouri was eligible for statehood?

The issue of if it would become a slave state or not, this led to the Missouri compromise.

The Northern States wanted to admit Maine as a free state. Therefore the south wanted Missouri to be admitted as a slave state. The controversy was that there was a rule that no slave states could exists above the imaginary line drawn at 36 degrees 30' latitude, and that was Missouri's southern border.