Intelligence is not a superpower

[Tanarii]

Do you think that had more to do with the TSR ruleset/DM guidance, the people you played with, everyone's age at the time, or some other factor?

Well now I know it was endemic in the community, not just D&D. But at the time, I recall it was being heavily pushed in non-TSR books by Seimbeida & Wujcik. In TSR, many modules and eventually most of 2nd edition splat was built around the underlying concepts in RPG-elitism. So yes, it was TSR, but whether they were responding to the zeitgeist or pushing it I don't know. (Eventually all this led to The Forge and Ron Edwards and GNS.)

Planescape with its very annoying berk-this and berk-that and steam-punk art would never have happened without it. And of course a desire to take the infinite and mentally shrink it into something recognizable and manageable and with a consistent theme for a setting, which both of those played into.