Are you kidding?
We are talking about two different eras, here.
Armor always looks different in other countries/cultures, and hundreds upon hundreds of years apart.
You honestly prefer the look of legion armor in Oblivion, to Skyrim's version, or are you just wondering why they changed it?
If you hadn't noticed, everything has been changed. Steel is different, irons is different, ebony.. all the way down the list.
Imperial armor is based off of Roman Legion armor.
Roman legion armor changed through the centuries like 100 times, in appearance.