As always, politicians using a law rather than putting any actual effort and money into solving the issues. Areas with high levels of road deaths (per capita) are typically rural areas where people flock there to drive like dickheads. Won't be affected by a 20mph zone in towns and cities.
Urban accidents are caused between clashes between different road users. That can be solved by effective urban design, but that's something that might take decades to implement, so isn't too appealing to a politicians who wants to be an instant hero. In the Netherlands, for example, many neighbourhoods have raised junctions which forces cars to effectively go over the pavement when they want to join the main road. It establishes is as a place for pedestrians, not cars, and creates a natural speed bump, so people naturally drive more carefully. Then add to this, the separated cycle lanes that remove clashes between cars and more vulnerable road users. I wonder how many people are injured because a cyclist is trying to get past a car that has parked in the cycle lane, or get around a bus that has to stop to pick people up. But that sort of thing involves far more work. As I said, it's much easier to just sign something into law and act like you've achieved something.