Board Thread:Lore Discussion/@comment-28142695-20160712153804/@comment-1600847-20160713134953

I think it has to do with the state of the Empire's economy. The full-bodied metal plate armor that the Empire had at the height of its power (Oblivion) is much more expensive than studded leather/chainmail chest, bracers, and boots (Skyrim). With the Empire in decline and only 3 provinces remaining, it might simply be too expensive to outfit the entire Imperial army in full metal plate.

But the most likely reason is Bethesda wanting to make each game feel different, thus having the Legion armor different in each game.