When I took the Hidden Wonders of World Showcase tour (I think that was what it was called) a few years ago, our guide commented on this. He said that originally it was a relatively fancy sit-down restaurant, but the number one comment/complaint from out-of-country guests was that when they think of 'American' food, they think of burgers/fries/hotdogs etc, not gourmet. So they changed it to counter service.
Not sure if that's the only/real reason, but that's what he told us.