WELL, I think it might have something to do with the Royal Guard vibe: professional and not someone you want to fuck with. A royal guard with a boob-window simply looks incompetent. The shade of yellow they used bothers me significantly more then anything else, it's super jarring and unpleasant.
Overall however, Fiora is pretty much the only champ I'm fine with in terms of the window, practicality-wise; looking back on dueling, normal clothes were worn, or just pants only to prevent cloth getting into wounds and causing an infection. (I mean, a guy walked all the way home after getting stabbed in the heart in a duel... and then died)