I actually think Gary was one of the originals who popularized it (he graded a ton of those CD Promos way back when). Ken Goldin also on that old video with the Neo File promos.
Most people were not grading cards. Outside of base set and a handful of Japanese promo cards, it was mostly unused even by serious collectors. I remember having to add a lot of the cards and categories when submitting to PSA. It started to become more populr around 2011 or so and then grew from there. It was only a matter of time.