maybe money has something to do with it, but the way the two websites are marketed/positioned definitely plays a role as well, as it creates a bias in the hosts.

couchsurfing is marketed as "a great way to meet locals", as a way to find new friends, so people who list their places on it are more likely to be interested in making friends with their stayers.

airbnb is mostly positioned as a "turn your home into a hotel" website, so many airbnb hosts operate it like a business and are not necessarily looking to make friends with the clients.