Cause I have a go pro and it didn't work great. You see Gopro cameras (Even the HD2 with improved lighting) don't work very well in low light conditions. He had the camera in a tube or the other fella looking through the scope. Not near as much light is getting into either. digital cameras work better for this.
That and the gopro hd doesn't have a great focus on it. It would be very hard to get it to focus onto the image in the scope.
Then the screen resolution isn't great. A good digital camera would have a much better screen,
focus easier, work with less light and would be cheaper (of course that depends on the camera and if it's waterproof etc.)
It might work on top of the gun but you won't have any zoom and you won't be able to see the enemies.