The bigger hole is that these things even exist in the first place. We know they have bloody floating tanks 50 to 60 years before Endor. Or maybe it the Empire is using the most inefficient design because it was the cheapest bid. Prequel aside you see floating vehicle all over the OT.
from overview for madhi19 http://www.reddit.com/r/StarWars/comments/2qlf6l/what_happened_to_this_atat_during_the_battle_of/cn7yozn