It's probably because they don't meet some California environmental regulation (like energy efficiency or toxic chemicals) or aren't certified to meet those reg

It's probably because they don't meet some California environmental regulation (like energy efficiency or toxic chemicals) or aren't certified to meet those regulations. California tends to have the strictest environmental regulations. Some models of cars are unable to be sold here for the same reason, although most are designed to conform to California regulations due to the manufacturers not wanting to miss out on the California market.