Because they are approaching a target that has pretty heavy firepower and they lack consistent cover, smaller vehicles like the bikes on Endor are not a good option. For something really big -- and therefore somewhat slower, though still faster than AT-ATs, there is the question of repulsors kicking up snow in its own face, which would make return fire difficult. It also might just crush the snow into water (it's fake science, so we don't know how it would react) which would make it very bad at dealing with deep snow.
"So why not just bomb the base from the sky?" Well a) I think they did but b) the armor issue and c) they had to reach the ground at some point. AT-ATs are dumb and tacky but, if you have access to all the general types of vehicles we've seen in the series, I think they are the best choice.
Because they are approaching a target that has pretty heavy firepower and they lack consistent cover, smaller vehicles like the bikes on Endor are not a good option. For something really big -- and therefore somewhat slower, though still faster than AT-ATs, there is the question of repulsors kicking up snow in its own face, which would make return fire difficult. It also might just crush the snow into water (it's fake science, so we don't know how it would react) which would make it very bad at dealing with deep snow.
"So why not just bomb the base from the sky?" Well a) I think they did but b) the armor issue and c) they had to reach the ground at some point. AT-ATs are dumb and tacky but, if you have access to all the general types of vehicles we've seen in the series, I think they are the best choice.