Cars made to be sold and driven in Japan (aka JDM vehicles, for Japanese Domestic Market) can not be imported into the US until they are over 25 years old. This is part of a series of import laws that American vehicle manufacturers lobbied for to keep foreign cars from dominating domestic marketplaces.

The US also has crash test safety standards that domestic cars must meet because a) safety is good, and b) people drive tanks like maniacs. Kei cars used to be pretty awful in crash tests, but have gotten a lot better in recent years.