There's between 6 and 10 original idea RPs on the first page of the casual and advanced forums right now, give or take what your measuring stick for "original" is. I think the biggest problem is it's not what some people want for [i]original[/i]. Not everyone wants to do fantasy or sci-fi. 

But yeah, fandom RPs will always invite people excited to write in them because they are [i]fandoms[/i]. If I could get a fandom for my old post-apocalyptic medieval fantasy RP that would be rad, but sadly I'm still waiting to hear back from Netflix on the filming rights /sarcasm.

Fandoms are a known thing, so of course, people are going to flock to that. Original RPs bear the cross of having to build interest from the audience's first glance, and that's a more difficult thing to do, especially when a lot of people find reading background lore a time investment they don't know if they should do.