Kentucky is famous for the race horses raised on bluegrass, like the famous race horse raising area in Ireland, the curragh of Kildare. I've read that the minerals in the pastures and hayland of Kentucky Bluegrass in central Kentucky is important to the formation of strong bones in the horses. Here is a bit about it from the USGS (US Geological survey) web site:
"Fertility of the residual soils in the Inner Bluegrass region is enhanced by the presence of phosphate in the Lexington Limestone. This area is the locale of the horse-farm industry of central Kentucky."

Because raising race horses is important to the economy of Kentucky and is one of the things Kentucky is famous for, Bluegrass is associated with Kentucky more than the other states where it grows.

Alice