In addition to being huge battle robots of death, the Imperial Walkers also boast a blind spot in excess of 300 degrees. The thing can't turn its head. Specifically, it can only turn its head this much:
This is not a minor issue. That up there is an AT-AT trying to shoot an enemy ship that's flying past it. That's as far as it gets; it's like a huge, muscular guy who happens to have his head frozen in a neck brace due to a crippling spinal injury. The only difference being that guy could still conceivably punch an enemy by swinging at him wildly. Meanwhile, all of the AT-ATs guns are glued to that non-swiveling head.
"Two targets for the price of one. Nice design, Kuat Drive Yards."