It has to do with the physics of a rotary hook. If the hook is spinning clockwise, the thread meets the needle in a different place when the machine goes left to right, than if the machine is moving right to left. The thing that helps is to rotate the needle slightly to the right. But... it still works much better going from left to right, especially when you're stitching a straight line.
Now if they could only make the hook spin the opposite direction when the machine moves right to left. Or... build an oscillating hook that can go that fast.