AT-ATs managed to get to the surface of Hoth - during the Battle of Hoth.
Why didn't the Empire send other forces in alongside the AT-ATs?
Two reasons to land the other troops:
Pretty much all modern Earth militaries learned early on that armored vehicles themselves are less effective than combined assaults (support by artillery, infanteyinfantry and air).
As it was, rebel air force (groundspeeders) very effectively opposed the AT-ATs, without being hindered by Imperial air support.
And Luke took down AT-AT because there were no Imperial stormtroopers to cover his approach on foot.
If you land all your troops - some of them could take the shield down during the course of AT-AT attack.
As such, sending in only AT-ATs first gives rebel forces more time to escape.