How did the idea of popular sovereignty in kansas and nabraska territories contradict the Missouri comprimise of 1820?

The Missouri Compromise had stated that all states above the 36 30 parallel would be free, and all below it would be slave states. This had established a begrudging peace between the North and the South, which lasted until the Kansas Nebraska Act of 1854, which repealed the compromise and introduced the concept of popular sovereignty in determining whether a state would be slave or free.