They couldn't afford it.
Not sure if that's still the case.
Also I think the more popular one's may have a fear of being affiliated with & advertising with other gambling sites which are illegal for now...
Yeah definitely not worth the headache for the big 2 to advertise in the same space as offshore books.
What do you mean by couldn't afford it? If you can't shed light on RX business dealings that is fine but I always figured for a site to post here there is no "fee" and more of an affiliate-based model based on %'s.