Final answer:
Missouri was deeply invested in whether Kansas allowed slavery because it affected political power balance, the spread of slavery, and the region's future stability.
Step-by-step explanation:
The main reason that Missouri was particularly concerned with whether slavery was allowed in Kansas relates to the balance of political power and the spread of slavery. Missouri, as a slave state themselves, was invested in maintaining a balance that would not tip against their interests. When considering the debates over Missouri's admission to the Union, an important aspect was Congressional representation, which southern states, including Missouri, enhanced through the "three-fifths" compromise.
Missouri's interest was especially acute following the Kansas-Nebraska Act of 1854, which applied the principle of popular sovereignty to the Kansas-Nebraska territories, overriding the Missouri Compromise and threatening to change the balance of free and slave states.
Missouri's involvement in Kansas politics, including the votes by Missourians in Kansas elections, further shows the depth of the state's concerns over the slavery issue. The potential change in regional dynamics, as foreshadowed by David Atchison indicating that failure to secure Kansas as a slave state would result in a domino effect losing Missouri, Arkansas, and Texas, further explains Missouri's vested interest. The violent events of "Bleeding Kansas" demonstrate just how significant the issue of slavery expansion was to the region's states, including Missouri.