Because Kansas is named afer the Kansa or Kaw Indian tribe and in Spanish pronounced Kansas. The word "Arkansas" itself is a French pronunciation ("Arcansas") of a Quapaw (a related "Kaw" tribe) word "akakaze" meaning "land of downriver people"

Arcansaw pronunciation was made official by the State in 2007.

In the early days Kansas was mostly part of the Spanish territory of Texas and Arkansas was part of the French territory of Louisiana.That is why one has a Spanish pronunciation and the other is French.

Here is an interesting oxymoron Arkansas City, Kansas is pronouced the spanish way Arkansas City by most residents of Kansas and a few pronounce it the French way Arcansaw City