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.
Tax is tax, money is fungible.
Why should I be taxed to fix road damage based on a metric that doesn't begin to accurately reflect my actual contribution to the problem?
Additionally, if the trucking industry had to raise rates due to a higher 'road maintenance tax' this may also make more efficient forms of transportation more popular, ie trains.
The fact that more fuel-efficient drivers pay less taxes is a bonus, since fuel use has negative externalities.
Personally, I don't understand why we don't have more flexible tariffs in place to secure against this kind of influence.