Inner doors should swing inward because in case of a fire or other emergency, you don't want to be trapped inside by something blocking the door that you cannot deal with. Even in something were preventing it from swinging open (like a rope or whatever), in theory the person could still get out because the door hinge would be located on their side and they could simply remove it.

Also because the hinge would have to be on the outside if swinging outward and thus not be securable. As the hung pins could be removed and door opened while locked.