Because open-cell is not a vapor barrier and most buildings here need one. In a livestock barn, a greenhouse, a wash-down area, a cold room or a crawl space, moisture passing through the insulation to the structure behind it is exactly the failure you are trying to prevent. Open-cell insulation costs roughly half as much per board foot and it has real uses, mainly interior partitions and sound damping. It just is not the right product for a humid, steel-clad or ground-contact building. If someone has quoted open-cell for your barn roof, ask them how vapor is being controlled.