From the NY Times...
"Zoe and her team almost never buy anything new — they borrow or they are given garments from nearly every designer. Some designers say no. A year ago, Zoe contacted Olivier Theyskens, then the designer of Rochas, about dressing Keira Knightley. He refused. “I was shocked,” she said after ending her phone call. “I honestly could not believe that he wouldn’t want to dress her. He’s a brilliant designer, but he doesn’t understand how the business works.”
Doesn't understand how the business works, or doesn't want to be associated with Zoe's particular brand of overexposed tabloid-pandering glam?
I'm going with the latter.