Because fuel taxes are supposed to be an approximation of how many miles you drive, aka how much wear and tear you put on the road infrastructure.

You put the same wear and tear on the road at $1/gal vs. $4/gal. Trying to create budgets against a commodity that rapidly changes prices as much as gas/diesel does would be rather difficult.

For example, in 2015 we'd be forecasting huge budget shortfalls for any planned road maintenance due to the unforseen huge drop in gas prices.

Fuel taxes are typically allocated towards road construction and maintenance. Fuel use is roughly proportional to road use, so it's roughly a tax that makes road users pay for their portion of road use.

The fact that more fuel-efficient drivers pay less taxes is a bonus, since fuel use has negative externalities.

Depends on the commodity... cigarette taxes, for example, or other sin taxes...

Personally, I don't understand why we don't have more flexible tariffs in place to secure against this kind of influence.