Because roads still need to be maintained no matter what's driving on them. Those costs won't change.
Um, the type of traffic being carried by and the maintenance cost of roads are *not* independent. The more and the heavier the vehicles on the road, the more damage caused, the higher the cost of maintenance.
You're better off increasing the gas tax itself which then hurts less efficient (and presumably heavier) vehicles while reducing the impact on more fuel efficient (and lighter) vehicles.
Less fuel efficient vehicles need not be heavier and more fuel efficient vehicles need not be lighter. I recently retired my old car for a newer car and the newer car is twice as efficient and twice as heavy.