Why? And some of this skins i have see other people using.
Inorder for a skin to be used ingame every player must have that skin downloaded to their PC for it to appear. If a single player doesn't have it, it will not appear and instead be replaced with vanilla. Therefore it's more difficult to get custom skins to work in 4v4 since their is 4 times the amount of players than that of a 1v1.