Because it would be morbid and inappropriate for tourists to gawk at a shipwreck.
same way there is a difference between photos of dead bodies undergoing forensic investigation, and those undergoing archaeological investigation.
I'm not really joking here, there has to be some sort of threshold and I'm not sure how it can be defined.
It is somewhat surreal that a historically short amount of time must pass before people can visit these areas without it being taboo. Some of them, in my opinion, are extremely important to humanity such as Auschwitz and Choeung Ek, and the latter happened during my lifetime.