1) Gas tax is a fixed value, not a percentage. It’s been politically next-to impossible to “raise” the federal gas tax (or state ones). So the fraction of gas price going to infrastructure has been steadily decreasing over the years (plus inflation losses).
2) Gas efficiency (miles/gallon) has been been climbing over time (those Priuses aren’t helping), so less money for infrastructure.
Corbett sort-of got around the problem by introducing a wholesale gas tax (and now has the anti-tax zelots screaming at him).
Fun fact: The Penna constitution has an amendment (§ 11) that prohibits the use of gasoline tax for anything other than “public highways and bridges”. I’ve occasionally wondered if this is the reason we’re stuck with those 35mph speed limits on state-maintained city streets.