The cost of death is extremely high. In urbanized areas, designing streets for even slightly lower speeds (say 40 km/h vs 50 km/h) has a huge impact on crash, injury, and death statistics.
I think a fundamental issue is that we prioritize motorized vehicular traffic over everything else. After decades of doing that, it seems absurd to most people that we should shift away from that, because they don't see that there are viable alternatives.