As BOB2 sez
Road damage- that is. what causes maintenance expense, is done by axle load/number of axles.
Look deeper and one should find a statement by a trucking association honcho saying fuel tax is the 'right' way. As B2 points out, that lays a lot of the expense onto drivers of automobiles.
And as others wrote, fuel tax is a dead end as all-electric-all-the time approaches.
But hey, who has the cash to talk early and often to legislators. TAA/ Teamsters?

Now then, in the event axle load does become the means of taxation, muse on the effects on us folks who must buy things, oh, say food,and other consumables, etc. We'll pay in that end, and more like our share.
But keep on musing on- What would an axle tax do to transportation on the whole?
Safer highways where the difference in vehicle size is reduced?
Less need to drive a big SUV to feel less like a bug on the road?

OOOO Here's an idea- what if rails were used for heavy loads and deliveries were made by lighter vehicles?? Because of that old bugaboo Economics

And as for "If you can afford a car, you can afford the registration, etc"
Well some folks, in this economy of looow paid jobs, hiiiigh living costs, scrape by to find an affordable place to live and drive that long way to a job while maintaining that used old car that was 'affordable'.

MUSE you Muthars