The younger crowd (30 and under) have in general less exposure and access to the skills and cars. Many schools have stopped offering shop , small engine, automotive classes. So for many unless you have a Mom,Dad, family member, buddy, group that is into wrench'n you have no clue about anything beyond the dashboard of a car(even that can be a mystery to some). For those people the only option is a newer vehicle that they can take anywhere for repair and maintenance. That does not even touch on the enviro/political hacks that are against everything old and do it yourself. CfC killed many thousands of older cars which was an intended goal for them. To many a car is just an appliance. So it is imperative for people like us to draw people to our hobby and take the time to educate them and instill that love of cars. This can be difficult because they will ask and do many stupid (to us) things, but we must resist the impulse to insult them or walk away from the opportunity to educate.
I taught my 3 kids about cars and made them learn and perform the basics. My garage is open to their friends along w/ my knowledge and time. Sadly sometimes even my $$$, but I love having them all around talking sh*t and getting greasy.
Sure this is true for some but it's bigger than that. Many of these guys that have cars like VW's, Subaru's and others modify and tune the hell out of them and do much of it themselves. I think it's a few layers.
1- Like ribbed said, financing.
2- Practicality. Many of the cars that are hot these days are 4 door cars. Much easier to justify to the girlfriend/wife or even ones self(if you're lame and afraid of life). They will also be more reliable as a daily and likely have a warranty if its still new enough. Guys like us have a daily, a G-body and maybe other older cars. These guys don't have the means(or the balls) to own more than one set of wheels.
3- The evolution of modding and tuning newer cars is still evolving. Younger people likely feel more connected that. It's more cutting edge to be front runner rather than look to nostalgia.
4- Computers. Our whole world is computers and you've now got an adult working population that lives and dies by them. Newer cars have sophisticated computers with a wide range of tuning ability. With all the tuning software and devices coming out each year, this makes them feel connected to the process on a different level. The things you do to a G-body to mod them are the same exact things you do to a 1967. Big v8, gears, transmission, carb/Fuel injection. And that FI is usually a more simple setup.