The problem is that purchase
The problem is that purchase of 2WD vehicles isn't random, and neither are fatal accidents. Fatal accidents require a certain amount of speed to happen, and are more likely to happen in good weather (you aren't to die when the roads are bad enough that you're crawling along at 20 mph). If you break down fatal accidents by state (and correct for miles traveled), you'll notice that (with a few exceptions) the states with worse winter weather have lower *fatal* accident rates than the states that get little snow...and while I don't have purchase data to confirm, I have to imagine 2WD purchases are much higher in those states that don't get worse winters. Minnesota, Wisconsin, Michigan all have death rates per 100 million vehicle miles traveled below 0.9; Florida, Louisiana, South Carolina are all above 1.4 (50% higher).
So it's more complicated than this data suggests.