Springfield in the Simpsons is supposed to be just about any town that you can find in the US but the inspiration for this is the hometowns of the creator Matt Groening which is from Portland, Oregon. Mike Scully is also from Springfield. This is probably the inspiration for the town’s name.
Some of Simpson’s loyal viewers say that they have realized that the series is based in Oregon all along but they did not want to make assumptions until Groening confirmed it. It is probably easier to create something based on what you have been exposed to. The setting is more comfortable that way.