To keep the balance between free and slave states coming into the Union.
The Missouri Compromise did not account for new land acquired from the Mexican-American War. To maintain peace between the states and in Congress, the solution was to keep states coming into the Union in pairs. One free and one slave state had to be admitted at the same time to keep the balance between the two regions. The Compromise of 1850 brought in California as free as long as a stronger Fugitive Slave Law was put into place.