Because California is a G10 country all on its own, and has a larger economy than the last 29 states in the US. They also buy approximately that percentage of cars. And geographically, most of their population is subject to this weather phenomenon called in inversion that traps pollutants close to the ground.
So, if almost half the cars you made were sold in a jurisdiction with high pollution controls (which is circular - the more cars, the more reasoning for pollution controls because of the public health impacts (which impact GDP etc etc)), how would you explain to shareholders that you were developing TWO pollution control systems for every make and model? As someone who holds shares in automotive companies, and who votes at those meetings, I'd vote your ass out in heart beat if you were to piss away money that could be dividends to make another system.
So in short, unless your state consumes as much as California, you don't get a vote in how the vehicles are built. I mean, what's your state do, live on federal subsidies to NOT farm land? Or farm corn to go to ethanol because of the subsidies? It's the same crony capitalism from the other side of the aisle.