Just a thought. One obvious difference to me is that the Mormons were expelled from Missouri as a result of state action. The militia was mobilized for that express purpose, and the governor issued an "extermination" order. In Kansas, on the other hand, the invading Missourians were primarily acting "on their own hook," and did not have the full strength of the state supportiong their effort. Also, expelling people from a couple of counties was likely a much easier task than occupying a state, even though it was lightly populated.