They pretty much already do outside of CA. Most cars sold in the other 49 states are built to meet Federal EPA emission standards. CA is the only state that is allowed to set its own emission standards since CARB predates the EPA's emission regulations. The other states have the choice of adopting either Federal EPA standards or CARB's stricter standards, but they can't develop their own unique standards. However, the EPA pretty much just copycat CARB's emission standards so the differences aren't big. Also CA is a huge car market that car manufacturers don't want to miss out on, so they are willing to jump through any hoops CARB sets up. Only good news is that most states don't bother to enforce emission standards as hard as CA does.