Because marketing departments have more sway than they should. These things come and go, 3D, "interactive movies," etc. It's a different format, "added value" is a term I imagine someone throwing around at a board meeting. It won't replace anything, just supplemental.
Besides, it takes a LOT longer to shoot in 3D. For each shot you have to set (or even pull) IO distance, convergence, and focus. You also need more light than you would initially think, which means additional g/e gear and crew. More cost per shoot day, and more days of shooting because of the added time to each shot. Even if the gear is relatively inexpensive (half-silvered mirror rigs with Sony EX-3's are common) the added production cost will be prohibitive for a lot of projects for quite some time.
It's hot now, and will probably see lots of play in specialty areas - sports for example, but taking over completely? Doubtful at this point, certainly not in 5 years.