I don't personally believe this is reality, but a while ago an affiliate stated "no dicks, no clicks" and as long as most affiliates believe that, they will not aggressively join and promote softer content programs.
I started
http://www.BoobieMovies.com specifically because I don't always want to see dicks in my porn. I don't think I'm alone on this, but once a belief is out there, it is tough to change the status quo.
I think affiliates are missing out on sales by limiting themselves to the "no dicks, no clicks" strategy.