Politically a new traffic lane is worth a lot more than maintenance. Even though people scream about potholes you don't get a ribbon cutting photo from a repaving project.
Since most roads in the U.S. were built we've had a massive increase in suburbs. Suburban infrastructure is extremely expensive compared to urban and rural. Suburbs (and the increase in big-box groceries and such) massively increase the amount we drive and the amount of wear & tear on roads. Per capita we drive twice as many miles today as we did in 1980 and almost 5 times as many as we did in 1950.
The low density + high mileage of suburbs means that the per capita revenue (property taxes and gas tax) to maintain all of this infrastructure is extremely low. By most estimates the gas tax needs to increase to over $1.50 / gallon to adequately maintain what we have. With the proliferation of electric cars though we'll realistically need to go to a wheelage fee based system of some sort. This will be good though since people will directly see the cost of their driving on a per mile basis — estimates are about $0.18 per mile to cover road maintenance ($0.07), law enforcement ($0.08), and environmental impact ($0.03). These are averages and will be higher in frozen states.