Can we get Ironstylus or a Riot employee from the art division to answer why they deviated from the concept art?
Because we do a lot - few things look as good in-game as they do in the concept art because of the view and angle. We're also really serious about clarity/readability, not having things blend in the background, being able to see the shapes at game-view, etc.
But most importantly, game development is about iteration and change - always, forever. Why they specifically chose to change this is something the skins team could speak to more, but my guess is the shoulder pad and color got lost at the zoomed-out camera view - the only view that matters when it's time to load up your skin and use it in game.
Full Metal Jayce's concept art is better than what it is right now...
What it was planned to be: http://leagueoflegends.wikia.com/wik...ce_Concept.jpg
An all blue Jayce doesn't seem to be very exciting.
But most importantly, game development is about iteration and change - always, forever.