However these rules came to be, from whoever lobbied for them in the first place, cars that are not specifically certified by a manufacturer for sale in the US cannot be imported and registered for public road use unless the car is at least 25 years old from month of manufacture.
Like how it generally works in Western countries nowadays, all new cars sold in the US are certified by the manufacturer with the NHTSA that it meets federal crash standard and safety laws and with the EPA that it meets emissions laws, and it's generally specific versions of cars that affect safety and/or emissions gets certified, hence even if a model is broadly certified for sale but a specific engine combination never did, that engine combination cannot be brought to the US and sold until the manufacturer runs the tests and shows certification that it meets the rules. All new cars sold in the US will bear a sticker saying something about that.
The federal safety and emissions laws are exempt as soon as a car turns 25 years old and thus can be registered for road use even if the cars did not meet original federal rules, hence why when some cult car that was previously unavailable in the US turns 25, there's a rush of Americans importing them.
The main exception is what's called the "show and display" law that allows certain "historically" or "technologically significant" cars to be imported for very specific, limited use. There's a list of those cars maintained by the federal government and to get a car on it requires and application and proving that a car does indeed meet that requirement.
Of course, even when cars are legal for import at the federal level, states can still levy additional requirements for road registration (and it is up to each individual state to issue registration and plates). California is probably the most famous example of this, and their rules above and beyond the federal rules make 25+ year old cars still very difficult to register there.