Why is it illegal to bring an import car from japan and register in CA, like the Nissan Skyline or S15?

It's all about US specs, the US wants cars to meet their specs. The only reason for that is it's what they want so if it means modifying certain things then you have too. If you want to drive it in the US. It does sound lousy because I know some want that Skyline out here but the states and govnmt makes the laws.