My thoughts on why they probable excluded motorcycles. The idea is to give people an incentive to buy these fuel-efficient vehicles. People who buy motorcycles are not buying them for their fuel efficiency, and would buy them anyway, so they don't want to give up any money they can keep if it won't help the cause.
That said, if people paid the real cost of driving per mile, rather than it being subsidized by various other taxes, the problem would take care of itself without government trying to "fix" the things they broke. But, that's probably true of all government activities.
When a government takes over a people’s economic life it becomes absolute, and when it has become absolute it destroys the arts, the minds, the liberties and the meaning of the people it governs.
- Maxwell Anderson