I guess nobody's gonna explain it, but- Arkansas and Kansas are pronounced differently because they're not from the same language. "Arkansas" is Siouan-derived Osage about the Quapaw ("-aw", see?) tribe, and "Kansas" is from the Kansa tribe. They each mostly kept their original pronunciation. Most of the states in the US are named from whatever local tribe lived there, with the exceptions being mostly Latin-derived - Pennsylvania being a double-exception mix of West Central German and Latin.