"“I don’t think that people understand that ambulances don’t carry blood,” said Jeffrey Kerby, who is chair of the ACS Committee on Trauma and directs trauma and acute care surgery at the University of Alabama-Birmingham Heersink School of Medicine. “They just assume they have it.”"
I don't know what's involved in blood typing, but I can't imagine an ambulance is equipped to do that so they'd need to stock all O negative, and that's going to reduce the overall supply as it's driving around in ambulances.
Also, it's perishable, so I'd think hauling around a bunch of O- would increase waste of a very valuable resource.