“International law of the sea” or some such BS. Actually, probably a more practical reason would be that a ship and its crew are subject to the laws of whatever country (or state) they dock in. There are a lot of places in the world where even a secured, locked closet with some unloaded, stripped rifles would get a master and crew imprisoned and a ship impounded, much less if a merchantman itself had, say, a couple of M2 .50-calibers or something similar on mounts along the sides.
Cruise ships have experimented with things like “sonic” weapons (basically big directional bullhorns that blast deafening sound to drive off boats trying to board) but those ships are generally OK because they’re faster. Container ships with small crews, like this one, are the ones that are really in danger.
}:-)4