Kansas and Arkansas both derive their names from the same tribe of Native Americans, the Kansa. Kansas pronounces their state's name phonetically, but Arkansas does not. Arkansas (pronounced Arkansaw) gets it's pronunciation from the French word Arcansas. Arcansas was the French pronunciation of the Quapaw word Akakaze, meaning land of downriver people. In the mid 1700, when the French word "Arcansas" first started to appear, the pronunciation of the last syllable would have sounded much more like -sa or -saw than -saas. In 1881, the Arkansas state legislature make the "Arkansaw" pronunciation we use today the official pronunciation. The reason that Kansas pronounces their name differently is probably due in large part to the lack of A significant French presence in the area that is now the state of Kansas.

This is probably more of a question for /r/asklinguistics IMHO.

I don't know much about Mid-western English (I've only read briefly about the why G.A. became dominant over transatlantic English in television). But here is something interesting on the evolution of Southern American English accents.