For the most part, only a relatively few people who weren't the standard "cishet white guys" audience got brought in one way or another. From there, it slowly spread via word of mouth, through friends or college groups. Slowly as in over decades, not just years. For a very long time, maybe you'd have a woman or two in your game, or a black guy, or maybe one of your players would come out as gay. But it was very much a personal table thing, because RPGs weren't targeted or, more importantly, advertised to a wider audience for a long time.