I don't know, maybe because commercial vehicles are exempt from any testing. Please don't try to tell me that an 18 wheeler is blowing water vapor and sunshine out its tailpipe. The nastiest oil burning hooptie piece of junk car is probably blowing less crap than a clean burning diesel. The smog laws are poorly conceived and ineffective at doing what they are supposedly intended to do. LA and Houston battle it out for worst air all the time. Cali has had their smog BS for 20 years and Texas does not do crap, and LA still is smoggier sometimes, go figure. IF the state of CA was really that interested in air quality and public health, they would be looking at everything, not just passenger cars when they are only a part of the problem. The role that private vehicles play in the produciton of smog shrinks every year, as they get the cars to be cleaner and cleaner. The people making the rules are in bed with the people they are regulating. Private citizens don't have anyone lobbying the legislature for them, and our representatives here are beholden to them. You may have heard about our Governor getting booted for this sort of thing, and more recently our Secretary of State resigned because he is a crook and got caught.

BTW, that law that recently became effective from that lesbian bed-wetting bitch lieber exempts 1975 and older vehicles. They don't check, and you can take off your Fuel Injection for Carburettors, remove EGR filters, smog pumps or Catalysts. They are exempt, means just what it says. For "classic" vehicles, a nebulous, poorly defined type of car, 35 years old and older, the visual inspection is deferred. They do the sniff test, but do not open the hood, so technically, if I can get it to run clean enough, I could put a dual carb monster motor in my bug in 10 years.