Most people I knew at the time (high school & college students) thought of ET as a kiddie movie and didn't see it because of that. And, that was one movie from the 70s through the late 90s that you cited, whereas I cited Big Bang Theory that has featured D&D probably a good dozen times or more, as well as an entire D&D episode on Community, and numerous mentions over the years on both Colbert and The Daily Show. Heck, a few years ago, my daughter was watching Wizards of Waverly Place and they featured a D&D game in one episode, too. Not to mention the very last episode of Buffy starting off with a D&D game. Since the late 90s, you've had the 3 Lord of the Rings movies, a Hobbit movie, 8 Harry Potter movies, I'm not sure how many Twilight movies, The Hunger Games, Game of Thrones, True Blood, The Walking Dead, Once Upon a Time, Lost, Grimm, Heroes, etc. All part of the fantasy genre and expanding awareness of the genre as a whole. Heck, the only fantasy on network TV in the 80s and into the 90s until Buffy came along was the D&D cartoon. (Hercules and Xena were syndicated shows that also came along in the mid to late 90s.) And again, it was a Saturday morning cartoon so was relegated to mostly kids watching it and therefore not part of the mainstream culture like it is now.
Also, I have been an avid collector of anything D&D related from the 70s until now. I have never once seen anything related to D&D in any department store (KMart, Sears, etc) or toy store (Toys R Us, Kaybee Toys, Child World, etc). Not saying it didn't happen elsewhere, but maybe distribution never reached New England/the Northeast? Heck, I would have done backflips if I ever did see D&D material in one of those stores because I could bike to the local KMart when I was a kid it was so close. The D&D stuff I found was always either in bookstores, comic book stores or hobby stores in the 70s, 80s and into the 90s.