In a word, fashion. In a few more words, anything that MBR says is absolutely essential and makes all the difference between a good bike and a bad bike must be bad. They say you can't get your hands in the right position with a flat bar, which makes no sense to me.
The thing that most surprises me is that the riser bar fashion has survived the move to long-travel forks. It's one thing to say you want the front end higher, but with 130mm forks it's high anyway. But some people who find their bars too high would rather flip the stem than fit a flat bar.