We thought that this was a secret that Matt Groening, the creater of the Simpsons, would keep close to his chest until death. We were wrong. According to an interview in Smithsonian Magazine with Groening:
OK, why do the Simpsons live in a town called Springfield? Isn’t that a little generic?
Springfield was named after Springfield, Oregon. The only reason is that when I was a kid, the TV show “Father Knows Best” took place in the town of Springfield, and I was thrilled because I imagined that it was the town next to Portland, my hometown. When I grew up, I realized it was just a fictitious name. I also figured out that Springfield was one of the most common names for a city in the U.S. In anticipation of the success of the show, I thought, “This will be cool; everyone will think it’s their Springfield.” And they do.
And there you have it folks.