Because the US government bought it and still owns it according to US law and that "shall not be extinguished by the passage of time, regardless of when the sunken military craft sank." The current US law:
http://www.history.navy.mil/branches/org12-12a.htm
I am also pretty certain that international maritime law also applies to ships and aircraft owned by a state the concept of 'sovereign immunity' no matter how long the 'craft' has been in the water, as long as it was not abandoned willfully by the state (dumped overboard as trash, etc). The salvage company Odyssey Marine Exploration is now being reminded of that because a treasure they found in the Atlantic may have come from the Spanish frigate Nuestra Señora de las Mercedes, sunk in 1804. If the treasure is in fact from a Spanish Navy frigate the Spanish government could likely claim it all though the company is arguing otherwise.
I think the only way to ensure revenue is to become a Salvage Lawyer. These things get tied up in court for years sometimes.