It's because the creator made those clothes with a buggy, unpatched version of Body Shop. See this Wiki link for more info:
http://www.sims2wiki.info/wiki.php?title=Game_Help:Nude_Sim_Bodies_BugUnfortunately there doesn't seem to be anything you can do about it other than getting the original creator to patch their Body Shop and re-upload the content. I've had to give up having tons of nice clothes because I don't have Pets. And I probably should have at least alerted the creators, but I've been too lazy. Don't be like me... :-|