You guys are overlooking a real basic idea. The outfits have to exist in Omega and Alpha because of the simple fact that the games will interact and be able to battle with one another. The only thing I can think of they might not change is hats unless they make the X and Y characters look like defaults, which still won't work given the different skin tones.
They might have programed in the various models for the XY protags but only the defaults for ORAS ones and not have customisation for those.
Besides, do the oras models exist in XY, so maybe they will just show up as defaults there too.