Because such a tax would be regressive that it would be beyond stupid to attempt, even at half that amount. Already government regulation, taxes, and fees, affect the poorest among us. Those hiked cost to register your vehicle, obtain a license, renew plates, and such, all hit that segment the hardest.

You want a carbon tax, put it where it belongs, tax vehicles that return less MPG and priced beyond average car price purchases of the middle class. That Vette over there, bam, 10k extra. That Ferrari, 50k. See that lovely Audi S5 sport coupe, tack on another 10k there too. Tax small planes, tax motorcycles beyond a certain CC and size, but never tax the fuel. To encourage a change in what is an acceptable method of transportation means taxing what you don't want more of, not punishing those who are least able to withstand it let alone are not causing it.