I'm no expert, but I think that making a window, with a proper view that reacts to the time of day and the weather, would essentially take the same effort to make as a balcony. Either way they need to make a new (if limited) outdoor area. The only difference would be that with the balcony, you could walk a foot out into it.

In fact, it wouldn't surprise me if the balcony was actually easier to develop, since you would have to click on a door and travel from your apartment instance and into the outdoor instance, as opposed to having the outdoor area in the same instance as the apartment. There's probably a reason why even inn rooms and NPC rooms just have white windows that shine or dim depending on the time of day.