How did westward expansion lead to the Kansas-Nebraska Act?

The Kansas-Nebraska Act and popular sovereignty

Promoters of a transcontinental railroad also pushed for this westward expansion. Furthermore, many in the South were growing resentful of the Missouri Compromise, which established the 36° 30' parallel as the geographical boundary of slavery.

Explanation:

one of the major compromises in the constitutional convention was between the small states and big states. the small states wanted each state to have the same number of representatives in congress. the big states wanted representation based on population.