Comment by ubernostrum

In theory, the US system is supposed to allow the states to act as laboratories for trying out different policies and seeing how they work out. But when the federal government passes a law, in an area where both the federal government and the states theoretically could exercise jurisdiction, the federal law often pre-empts any state-level laws.

California's clean-air efforts were one of those laboratory-style experiments, though, and were deemed worthwhile enough that, when the federal Clean Air Act was drafted, it was set up to allow California -- the only state which really was experimenting in this area -- to obtain waivers allowing it to continue exploring its own policies. Other states have the freedom to choose to adopt California's standard, or the federal standard.

Also, the Clean Air Act is a bit interesting in how it works. Rather than applying for a waiver and having to hope it's granted, California is basically entitled to the waiver unless the EPA can show certain conditions have not been met:

https://www.epa.gov/state-and-local-transportation/vehicle-e...

Which in turn means that a President who simply dislikes California is not in a position to deny the waiver on a whim. Trump would have to show that the conditions for the waiver were not met, and would likely have to prove it to a court, rather than just give the order to an administrator who's beholden to him for employment.