I don't know much about the business of boxing in the US because I am an Asian people. Someone please give me some knowledge. Stop insulting me or say YDKSAB, OK?
There aren't a lot of boxers who have exclusive deals with television networks. Canelo Alvarez and Gennady Golovkin have signed deals with HBO to fight solely on that network. (But HBO has let Golovkin fight off HBO if they aren't interested in showing a particular fight.) The only fighter I can think of right now who has an exclusive deal with Showtime is Floyd Mayweather. Most fighters who appear on Showtime have done so because Showtime almost solely buys fights involving fighters who had deals with Al Haymon. But the fighters themselves didn't have deals with the network. At the moment, I believe only Mayweather (Showtime), Alvarez (HBO) and Golovkin (HBO) have exclusive deals with those networks. So, really, there isn't any network blocking any fight from getting made. The promoters and managers of fighters tend to be the people who block certain fights from getting made because they don't want to work with or share revenues with certain promoters.