I just don't have the time to learn many new games well enough to play them, let alone GM them, any more. I'm 53, and have work, a house to maintain, family, and other non-gaming activities that all require my time and effort. Time for gaming is very scarce. I'd rather spend that time playing games rather than learning yet another new and different system -- especially a multi-hundred page set of rules.
I can run TSR D&D, Classic Traveller, Call of Cthulthu/Stormbringer (and other BRP style games) and Marvel Superheroes almost without having the rule books at the table, so why not run them rather than spend time learning yet another new system? I can make an exception for very rules-lite games, but other than that I just can't make myself bother.
This is true for playing as well as running them. If the players really need to buy the book and learn the rules to play, then I really don't have the time. That's what I like about many old-school games, you could play without reading the rules, let alone learning them or "mastering" them.
I know this isn't what the gaming industry wants to hear, but I suspect a lot of people are in the same boat I am or will be as they age and acquire more and more time-consuming responsibilities. The constant churn of rules may not be in the long term best interests of the gaming industry even if it does produce profits over the short term. Think about it, I could go to the store and buy the same game of Candy Land, Life, or Monopoly I played (rules-wise, at least) as a child and play them with my niece and nephew. However, with the exception of a very few games like Call of Cthulhu, I can't do that with the RPGs I played in the 1970s and 1980s. Even staples like D&D are so different (rules-wise) now that they might as well be different games.