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.
Asked about keeping mum on the town's home state in past interviews, Groening said he didn't want to ruin it for people. No word on why he decided to come clean now, but my guess is it has something to do with wanting to ruin it for people.