Because it's imperative that the fastest shifting on the planet through all gears is the most important aspect of choosing shifters. How well do you do when you come up to a nice kicker and wanna tweak that sucker out in the air? Lot's of "bar english" will cause your GripShift setup to shift. How 'bout a tall bunnyhop where you really need to torque forward on those bars? I'm not moving my hands out and off the shifters just to hop up on a ledge or something.
But saving 2sec on shifting would probably be a benefit somewhere in the MTB world.