Speed restrictions.
Kei trucks were designed for use in dense Japanese cities, which is why they also work in European cities. They are nimble but have a low top speed. You’re not going 70 mph around a street corner for instance.
It would work in places like NYC for the same reasons, but remember that most of the USA is suburban or rural. You need vehicles that are capable of going fast if you’re going to get on a highway.
A possible workaround is to have a separate class for these, like mopeds or scooters, which are road legal but are not highway legal.
Are we going to completely ignore the absolute insanity of arresting someone for cheating on an exam.