Why not higher mileage standards faster?

For the US, higher vehicle mileage standards will be gradually phased in. The excuse historically is let the market decide. Now the market has decided SUV's and other low mileage vehicles for the most part are selling poorly. It's possible to build an SUV with higher mileage as a hybrid, but it costs more currently. I would guess if enough production was made the extra cost would come down significantly. So it's a chicken or egg question.