You are completely correct. I noticed that too, but it's more in the recent years. The older posts on the kinkmeme have more character prompts and less RPF. Which is a shame because I don't care about fake stories about real people and want to read about my favorite characters. But those stories are now so few.

I'm thinking it might be because the gloss of the characters is gone after 6 years. Meanwhile, through cons and interviews, the fans have gotten to know the actors better and created some sort of fantasy world about them. Maybe that's why they now like to write about them more. It's a shiny new toy in an otherwise old fandom.