If a car company guaranteed a specific vehicle, they'd probably have to charge a premium and have stricter cancellation policies (which most don't have at all now). With SUVs in particular, I find pricing is all over the map if you try to reserve one in advance and, as you say, it's not guaranteed. I find that I have the most luck just using National's Emerald Aisle which lets me pick the car I want when I get to the airport. But it's still not guaranteed--though I've always been able to get an SUV when I've wanted one.