Because is it rural. Farmers don't like having corners of their fields clipped to facilitate round intersections. And roundabouts have to be made large enough to facilitate multi-trailer trucks, not to mention farm equipment. Plowing them in the winter is also a real hassle/cost as opposed to strait-through square intersections.
Search "mini roundabout" [1], although these are mostly used in urban areas. In Britain, junctions in rural areas usually just have "Give Way" (i.e. yield) signs for the non-major road.