Because they helped strengthen their industrial-based economy
The Kansas–Nebraska Act of 1854 was an organic act that created the territories of Kansas and Nebraska. It was drafted by Democratic Senator Stephen A. Douglas, passed by the 33rd United States Congress, and signed into law by President Franklin Pierce. Douglas introduced the bill with the goal of opening up new lands to development and facilitating construction of a transcontinental railroad, but the Kansas–Nebraska Act is most notable for effectively repealing the Missouri Compromise, stoking national tensions over slavery, and contributing to a series of armed conflicts known as "Bleeding Kansas".
The United States had acquired vast amounts of sparsely-settled land in the 1803 Louisiana Purchase, and since the 1840s Douglas had sought to establish a territorial government in a portion of the Louisiana Purchase that was still unorganized. Douglas's efforts were stymied by Senator David Rice Atchison and other Southern leaders who refused to allow the creation of territories that banned slavery; slavery would have been banned because the Missouri Compromise outlawed slavery in territory north of latitude 36°30' north. To win the support of Southerners like Atchison, Pierce and Douglas agreed to back the repeal of the Missouri Compromise, with the status of slavery instead decided on the basis of "popular sovereignty." Under popular sovereignty, the citizens of each territory, rather than Congress, would determine whether or not slavery would be allowed.
