couchsurfing = free; airbnb = some money. some people wont want to host someone in their house without compensation.
While this could be painful in the short-term, it would help them achieve the ubiquity that they seem to want.
We didn't feel any obligation to hang out with the host, bring gifts or stay in touch, or to accommodate our host or anyone else at our home -- which made things a lot easier.
I think it's more correct to think of AirBnB as cheap hotel, not premium couchsurfing.