Disneyland is "family friendly." Kids love it. Their parents at least tolerate it, and a lot of them even enjoy it; the point is something that the whole family can enjoy together. It's fun, it's exciting, it's safe.
Teenagers hate it.
Dungeons & Dragons, on the other hand, has always been that weird, slightly dangerous game that your older brother played. In reality, it isn't actually any more dangerous than Disneyworld--a good deal less so, I'd say, when you start looking at what it teaches kids and the kinds of skills and attitudes it encourages--but man, do parents ever love to flip out about it. Even if yours didn't (Mine loved it; even with no more than a vague idea of how it all worked, they could see that it involved math, imagination, and social interaction, so they were all for their shut-in daughter getting involved with the hobby.) there was always that possibility that some do-gooder adult would try to make a fuss over it, which would have been the absolute height of teenage glory.
Even more important than that, I think that Grognardia is right when it suggests that D&D has always done best when it's been branded as an "adult" game, even if the true audience is somewhat younger than that which the box describes. That was certainly the impression I had of the hobby before and as I entered it. All the other D&D players I ran into around that period were adults, friends of my parents or parents of my friends, people who played D&D now or had back in college.
Which isn't to say that everything the OSR publishes should be Carcosa. Whether it's described as "for adults" or not, the vast majority of the material that any fantasy adventure game produces will, and should be, perfectly fine for kids--the weird, smart ones who'd be interested in such things, anyhow. But having Carcosa, a succubus illustration or two in the Monster Manual, and even the occasional porn star as a part of the scene can't, I think, hurt, in giving D&D that bit of an edge that it needs to be really successful as that adolescent entertainment that it's always been.
Maybe, in the long term, D&D is destined to be Disneyland. I've certainly heard tell myself of more than a few guys playing D&D with their kids. But personally, I can't see that as a long term central focus of the hobby. Thirteen years old is just too good an entry point, for a whole host of reasons. And thirteen year olds, by and large, aren't too interested in Disneyworld, in playing games with their parents, in safe. Their parents don't want them involved in anything that's actually dangerous, naturally, but that's the real genius of D&D. You're hanging around in your parents basement, eating chips, and learning a bit about reading, acting, and statistics, but overhear a few stories about The Book of Vile Darkness in your FLGS and you imagine that you're doing something edgy.