The problem is that other countries have seen nearly identical trends in vehicle market share trending towards larger vehicles and have seen sustained declines in pedestrian fatalities. John Burn-Murdoch went deep on this in the FT a couple of years ago (archive link at bottom).

> Most of the explanations commonly put forward for why US roads remain so deadly focus on broad structural factors such as vehicle size or time spent on the road, but a review of the evidence suggests this may be mistaken. Last year’s improvement is a case in point. Two reasons often cited as key causes of poor US performance both worsened: the total number of miles driven by Americans increased, and US cars continued to grow larger. Yet fatal collisions still declined.

> Adding to the evidence that this is not a dominant factor, car sizes in Canada, Australia and New Zealand have traced similar paths to the US without resulting in a spike in fatalities.

> Another theory is that the rise of homelessness in the US may be pushing pedestrian deaths higher. A recent study found that there had indeed been a marked rise in traffic-related deaths among the homeless, but this, too, can only explain a small portion of the overall rise.

> Instead, an underrated factor seems to be not American cars but American drivers [...] The determining factor seems to be different attitudes to safety, with Americans twice as likely as Canadians or Europeans to say they find it acceptable to use a phone while driving.

https://archive.is/Lggyg#30%