I believe this is the explanation for the solid outer walls on the balconies of certain ships and not others.
In many cases, the solid balcony walls are for structural reasons, and I believe that is the case on the Voyager-class ships. Because of their extreme height above the water line, the Voyager-class ships could not be fully constructed of steel because they would become extremely top-heavy causing them to roll excessively.... or even all the way over. To solve that problem, the lower portion of the ship is made of steel, but the upper portion is made of aluminum, thereby making the bottom heavy and the top light, solving the center-of-gravity problem. As for the balconies, I believe at least some, if not all of those decks are made of aluminum, and to make the ship rigid they needed to build more of a "web" around the outside of the ship. Having all that space taken up by glass...which is essentially open space from a structural perspective... would not provide the rigidity needed on those outer walls.
One recent exception was the Queen Mary 2 -- it was built entirely of steel to better deal with the stresses of North Atlantic crossings and to make it last longer. To counteract the top-heaviness problem, they built it much wider. The QE2 has 5 upper decks made of aluminum.