Because this whole Met Gala is a thinly veiled ritual. Everything is supposed to be the opposite of what it should be. It wouldn't be ridiculous enough to put the transvestite in a normal suit with mild alternations to make it fitting. No, put her in an oversized suit to increase her perceived victimization, to forecast just how far away from normal this is; and how it's never going to look right. Not good enough, put her in ridiculous overpriced sneakers.
Protip: Nearly all fashion is dictated by homosexual men or old women. Two groups that HATE the natural beauty of young women. Whenever say "society makes women have to dress like xyz" remind them that it's almost never straight men that work at the tops of these fashion companies.