[Cap'n Crunch]

They changed it because they are trying to hide where the Simpsons actually live, which is pretty much no where in the U.S..
On the one episode, Homer worked on the oil mines in West Springfield and almost dies, but Barney saves him with the helicopter. Lisa points out a map of Springfield and it shows Springfield next to West Springfield. She then says, "West Springfield is three times the size of Texas!" Therefore, the don't really have a state where they live.