Lots of high speed chunks of metal = major carnage
40 000 dead people per year in the US can't be wrong.
When driving in downtown TO, (I have done it from time to time), I rarely reach 50 km/h. 20-30 km/h is more typical (about the same speed as a bike). Lower speeds = lower fatalities.
If you look at the peds and cyclists killed in Toronto, they tend to die in the suburbs too - roads are wide, speeds are high, cars aren't looking for em. This is despite the fact that there are a lot more peds and cyclists downtown.