It looks like the Imperials are not meant to come out on top nearly every time, much like in the movie. One fan reached out to DICE Community Manager Matthew Everett asking:
He received this reply:
Walker Assault places you in a 40-man recreation of the Battle of Hoth, in which the Rebels must destroy Imperial Walkers (by calling in for Y-Wing support) rather than hold out for an certain period of time. This becomes preeeeetty difficult to do, given the fact that the Empire not only has access to the AT-ATs I mentioned, but the much more manueverable AT-STs.
Regardless of the issues, I still found myself playing the Walker Assault mode of the Battlefront beta much more often, simply due to how many different things I could do and how incredibly immersed I felt on Hoth. But hopefully, the Rebellion will have a fighting chance when Star Wars: Battlefront launches on November 17th.