Well what ever is the real reason here is the future and the future is now.That's not quite the point. Rather, how many folks have a portable Blu Ray player or a Blu Ray player in their car or bedroom? But many do have DVD players for such. If you're going to sell Blu Ray, you'd also better keep that huge base of DVD customers happy.
jwm
http://hothardware.com/News/Universal-To-Ship-BlurayDVD-DualSide-Flipper-Discs/