When Luke surrenders himself to Vader, we first see an AT-AT approaching the landing zone. This fearsome vehicle wreaked havoc and desolation on the icy battleground of Hoth. Despite its efficiency, this AT-AT was not seen again during the Battle of Endor. Why is this? Did Lucas simply forget it was there? Was it unfeasible due to the forest terrain?
The dense forest would have made it unusable for the battle. Also the cannons are too powerful to use without risking the troops or the bunker.
The latter, it seems. Note that the AT-ST was seen once in TESB on Hoth. I think part of it was also that the AT-ST could be more unstable as it had two legs, which would allow the Ewoks some advantages that they did.
I think it was there for intimidation purposes. If you're 100 feet up in an AT-AT, you're not going to see what's happening on the ground in a heavily forrested world. However, if you're in a 50 foot more agile AT-ST, you'll stand a better chance in combat.
Purely for non-urgent mass transport, however, the AT-AT may have been more efficient on Endor than a ground-based vehicle. I think a lot of the Imperial military technology (the AT-ST in particular) was deliberately meant to be demonstrated as being unsuited to combat in the forest environment of Endor, anyway, that was sort of the point. The Empire simply didn't anticipate guerilla warfare from the Rebels being a serious problem, let alone the planet's natives - they were big, they were strong, they were THE EMPIRE. It was the same overconfidence which led to DSI being destroyed by a one-man fighter.
In-universe, yeah, the AT-AT just isn't practical in the close quarters fighting. As for an out-of-universe explanation, I'm just speculating here, but I believe the AT-AT props were either waist-height or shoulder-height, and so for the Hoth scenes you could just use the studio floor with some fake snow to create the illusion of these giant mechanical monsters. With Endor however, I believe they were filming in an actual forest...so if you bring your little waist-height props and put them next to these gigantic trees, yeah that wouldn't work.
So, all the AT-ST models filmed in the forests of California were full size walking biped mechs that were destroyed by felled redwood trunk traps?
The models weren't all that large, as can be seen here: I don't think that there was much difference in the use of models. They could have put both into the forest without much trouble. I think the in-universe answer of them not being particularly suitable for forest areas sounds like the most logical answer for the movie-production as well, they probably felt that it wouldn't fit into the battle.
They heard Luke was in town. You know, the dude from Hoth? So they said **** it, send the Scout Walkers.
I always like the scene of Luke's surrender.The acting,cinematography,visual effects and the AT-AT of course.