The problem isn't that the AT-AT's were susceptible to air strikes. Their armor was far too thick for the T-47 Airspeeders' blasters to penetrate. However, because of their long legs, tripping would be a hazard, as demonstrated by the fact that they were essentially taken out with tow cables. Meanwhile, the AT-TE had a better range, and was much closer to the ground. I don't remember their armor situation, but they were actually very good in combat.