Because
it allowed citizens to decide whether they had slaves or not: the law
passed in the United States, in 1854, was enacted to create the states
of Nebraska and Kansas, which, in addition, were forbidden to have legal
slaves by the Missouri Compromise of 1820, thus,
with the Kansas-Nebraska Act, the Missouri Compromise was annulled,
without achieving that Kansas attained a legal slave state, allowed the
citizens of the newly formed states, to have votes to decide whether
slavery was allowed or not. The Kansas-Nebraska Act was the precursor to the Civil War of 1861.
