1970s: Dungeons and Dragons is released, leading to...
-The hobby as a commercial business beginning and eclipsing wargaming
-Zillions of fanzine-level imitators
1980s: Increasing Professionalization and Popularity of D&D, leading to...
-Lots of new players
-Younger and broader audience
-Games that aren't D&D become commercially viable and professional
-Nearly every genre extant as of the '80s has a game. Every subgenre too (in sci-fi for instance we have Traveller, Cyberpunk, Robotech, Shadowrun, etc)
1990s: Video Games Or Maybe Just Industry Decadence?
-Are video games why RPGs become less popular? Or maybe it was just the fad passing
1990s: Vampire: The Masquerade
-Wayyyy more women show up
2000s: The Internet and Cheap Color Printing
-Lots of little indie games
-Acceleration of communication and production in fan-products
-Easier path from fan to designer
2010s: Crowdfunding and Communities
-Easier path from fan to publisher
Mid 2010s: 5e, Stranger Things, Critical Role, '80s Teens Having Teenage Kids
-D&D becomes incredibly popular again
There are also factors I don't know anything about (how distributors have handled games, for example). You can argue about other significant game-changers around the edges but I'm more interested in the takeaways from the information we have as a whole.
Mine so far are:
-The only things that've moved the needle so far are: big external factors like new technology and big products with new content/presentation
-Did D&D's '80s competitors--Games Workshop, FASA, Palladium, Chaosium--do something right that so far has evaded today's non-WOTC publishers? Or were they just taking advantage of the fact that there just wasn't a game for x yet?