Looks like this fish was caught with a guide service from a local outfitter -- most if not all of the outfitters have "catch and release" policies. They would not permit a client to keep a fish, especially a world record-sized fish. They would want to release it to provide an inducement to future clients to catch the fish again.
Besides, can't taxidermists nowadays provide a mounted fish simply based upon some photographs? That way the trout can live to fight for another day while the client gets his mount -- a "win-win" situation, don't you think?