The correct answer to this open question is the following.
The admission of California as a free state threatened to upset the balance of power and violate the Missouri compromise.
In a time where northerners and southerners fought for power and control, California was admitted to the Union as a free state on September 9, 1850. More differences were generated between both sides regarding slavery. Let's not forget that the Missouri Compromise of 1820, had created legislation that admitted Missouri as a slave state but also admitted Main as a free state, in order to balance powers.
