Occasionally it can happen due to a bug or a timing issue on the client side (see the clock/profile swap methods of playing stuff solo). In this instance, however, it's very much due to the downturn in player numbers as I've often been lucky to see more than 3 other players in the tower at any one time. On the plus side, even the content creators have stopped trying to pretend it's not happening and that everything is fine, so 'maybe', Bungie will finally start paying attention. (I doubt it, though)