I have met and befriended three different people in the last decade that are poly. I even married one of them.
All in the Central Iowa area.
I think part of the lack of poly is not knowing/understanding the term; one of my friends just didn't know there was a term for the way she thought. And the other is fear that friends or family will react poorly. Sadly, the mid-west isn't the ideal place to be unique when it comes to fundamentals like family structure or sexual orientation.