I don't know, it kind of makes sense to not have any Imperial ground troops in this battle. The Empire has these huge walkers that are almost impossible to take down, so just let them stomp their way in and take out the shield generator so that Vader & Co. can land. Why risking losing men at this phase of the battle?

Sure, they probably didn't expect snowspeeders to be able to knock them down with tow cables, but they could easily stomp and pick off any Rebel ground troops in their way, whose blasters aren't going to do any good.

[Vader to infantry:] "You boys just sit tight. We'll let the walkers do the heavy lifting, then you can clean up what's left of their ground troops. Oh, and if you find R2-D2, take him out. I have a feeling he's going to really muck up my plans later."