The answer is a bit complex. It started because several years ago, professional divers (usually veterans from elite military units) used their left wrist for the pressure gauge, therefore the watch moved to the right hand. Later on, it became a "signature" symbol - if you were a diving watch on your right hand, it means you are probably a diver, probably from elite military units, probably a "macho".
Today it is only a tradition. There is no operational need for that (as diving watches also include preesure gauge built in).
Omer