I love the comments section on all of these 'millenials aren't driving' articles. They generally seem to try to summarize everything into a solid nexus.
1. It's the economy, stupid.
2. Kids these days are lazy: their parents are driving them around anyway.
3. So much student debt, like OMG!
1 and 3 have some merit, but I think that it's a bunch of things.
For starters, my 8 year old comes home and talks about saving nature and about how cars emit gasses that are bad for people. He doesn't get it from me; I'm car free for the money, not for the environment. A lot of the younger adults have been raised in a world where global warming has always been a thing.
We're increasing population more quickly than we're building roads. Additionally, younger adults are more willing to try new things. No 40 or 50 year old is going to stop what they're doing and reassess something as major as car ownership. But when a 20 something moves out of the house, you can bet that they sit down and take a good hard look at exactly what they need. This means that in a world where more people are taking up space on the same number of roads, there's a greater likelihood that somebody who's actually assessing their needs is going to say that a bike or bus pass is enough.
Delayed licensing. More states aren't allowing kids to drive by themselves until they're 18. This means that they go through a major phase in which they learn how to be independent adults without cars. Put another way, cars are no longer a defining characteristic of teen's lives until they are legally an adult. By then, they've already learned how to get around without them.