The main reason is we don't have a regulatory classification for a car that doesn't go to highway speeds but isn't a Low-Speed Vehicle (LSV), and there are safety concerns biased toward those things being on highways when they ought not be on a highway anyway (just like, for example, a bicycle).
Also from the Bloomberg article:
> The absurdity of deeming minicars unsafe while ignoring the danger of gigantic SUVs and trucks reflects a tragic blind spot of US car regulations, which have for decades conflated “car safety” with “car occupant safety.” The result has been an arms race of vehicle size, as well as a refusal to allow Americans to enjoy the myriad advantages that minicars bring to urban residents across Europe and Asia.
So they would fail high speed crash tests, but also wouldn't likely be in high-speed crashes because of the lack of high speeds………