I spoke with an ol war buddy who served in an AT-AT during another battle (sorry, for his security i cant say which battle) this was basically his response:

During that period in time and space, Imperial forces were going thru cost cutting measures after the loss of the first Death Star. There was the huge debt that had been incurred during its actual construction and then payments to all the broken family's that resulted from such a massive loss of (brave) life who were aboard the space station at the time of its demise.

One way Imperial Commanders could save cost's deploying AT-AT's was to have them built without armor protection to the top of the vehicle.

It was determined that Rebel craft were not capable of dive-bombing, it was safe from attack in this area and it was therefore an unecessary expenditure to have armor there.

case closed, move along!