First off, this is an excellent topic. I think things are improving. I don't have any real numbers, but I just feel like I see more pen and paper RPGs at major book stores these days, than say ten years ago. But I hear what you are saying.
When I was a kid in the early 80s, it felt like D&D (and maybe fantasy and sci fi in general) was all over the place: in movies like ET, at toy stores, on television, etc. Maybe that was just a special moment in time, when it was just more cool. Maybe it was also before D&D had really been branded as a game for "nerds and geeks" (if I were to be honest, I would probably have to say I am a geek myself). In my mind that is probably what makes it a harder sell now. I don't think it is that people wouldn't enjoy playing if they gave it a fair shot, they just don't feel comfortable making the plunge into gaming, because they associate with being uncool. Maybe if we experience another cultural shift (a bit like we did in the 90s) when being a little on the nerdy side is hip, there will be more acceptance of gaming.
I saw the author of Fantasy Freaks and Gaming Geeks, Ethan Gilsdorf speak recently, and he had some interesting things to say about gaming culture and how even among different types of games (LARPING, Table Top, Online, Card Games, etc) there are mental divisions and hierarchies that make a person from one kind of gaming hesitant to cross over to another type. Probably a similar mechanism at work.