The emissions roll back deal has the devil in the details.
The main reason the push back of California emission standards is getting attention, is because more and more people are buying SUV's and trucks instead of glorified go-karts that get 50 mpg. The whole emissions standards deal with automakers is that it's based on the average of all vehicles sold. They "could" more easily start reaching those higher mileage/low emission standards if more people chose little econo-karts.
But most people are over that eco-fad, with gas prices still relatively low, by long term inflation standards, people want safety, comfort and luxury for their new car dollar. Ya, there's still the Prius buyers out there, but not in ever increasing numbers the enviro-do-gooders had hoped for.
So, automakers are topping out meeting the yearly increases of overall fleet average in higher mileage/lower emissions. Squeezing more mileage/lower emission is pretty tough on trucks/SUV's without compromising strength/durability and power needed for their weights. Can't force people to buy those econo-box cars that get 30-40+ MPG. How many here would CHOOSE to buy one of those over a truck, SUV or cross-over?
That's why California is getting it's ass handed back to itself. The state would love to force higher mileage econo-box vehicles on its public, and that would be fine if California made them itself. Automakers have come a long way with higher mileage/lower emission cars, but fewer and fewer are opting to buy them. Electric/hybrids are helping with the fleet averages, but those numbers are still fairly small percentage in overall sales.
Many thought we'd be further along with that trend, but it's still lagging nowhere near what they'd hope for by now. Automakers were caught off-guard, but in reality are relieved they won't have to push high mileage cars on a public that doesn't want them (yet).