The Mexicans overthrow a person trying to set himself up as the new king or emperor of Mexico and practiced what they preached, I guess. They let the Central Americans vote on whether or not to stay and besides Chiapas they did not. Might just have been the upper classes who could vote or something though. I am not too knowledgable. The area wasn't that valuable back then anyways, probably. But since Central America had not been part of the same kingdom/colony as Mexico during the Spanish Wmpire, I would say there was no real emotional or administrative reasons to keep them on.