They liked it because it could have allowed new slave states.
Kansas-Nebraska act
The Kansas-Nebraska Act also led to "Bleeding Kansas," a mini civil war that erupted in Kansas in 1856. Northerners and Southerners flooded Kansas in 1854 and 1855, determined to convert the future state to their view on slavery.
by proposing these territories be allowed popular soverignty
Slaves
Kansas and Nebraska
Kansas and Nebraska were created after the Kansas-Nebraska Act of 1854. The reason for this Act was to open new farmland and create a Transcontinental Railroad.
Stephan A. Douglas proposed the Kansas Nebraska Act in 1854.
The Missouri Compromise was effectively ended by the Kansas-Nebraska Act, however since there was still turmoil as to the "Bleeding Kansas" dispute, it was thought that the Kansas-Nebraska Act would be shortly overturned. The Dred Scott decision by the Supreme Court further strengthened the elimination of the Missouri Compromise and the institution of slavery north of the Mason-Dixon Line by ruling that slaves were not able to take cases to court.
Stephen Douglas
The Kansas-Nebraska Act of 1854 called for "popular sovereignty."
The commonly accepted view in Congress was that left to themselves, the people of Kansas would follow the lead of Missouri and become a slave State. In addition, it was believed that Nebraska would be a free State. Basically the Kansas - Nebraska Act left the decision of slavery to the people. In May 1854 Congress passed this Act.