I have wondered about this myself for a few years and the only real answer I came up with that makes sense is that it is more socially acceptable in more places to not be able to drive. Sure there's been a bit more re-urbanization by people who would have otherwise been car dependent, and public transportation has actually improved in many places, if not nearly enough. Of course "don't got no money" and the related "schools don't teach driver ed anymore" are contributors, but when I was 16 people basically did anything they could to get themselves a license and, of course, a car. It was the top life priority. I can't remember ever knowing anyone in college or graduate school (except maybe some foreign students who used their stay in the US to get one) who didn't have a license.
So, yes, there are some financial and other incentives at the margins, but I really do think it's largely cultural. You can do it and not be a "total loser." Those things are related, of course - as the financial and related incentives tip more people away from driving, it becomes more socially acceptable - but I would guess that the real driver is social acceptability.
