I am assuming because it took 2 years to get to this point. With SOPA and PIPA in place there is no need for a takedown involving the federal authorities. Copyright holders can achieve the same much more quickly by using PIPA and SOPA to starve them of cash and audience.
Or in the case of foreign sites like Megaupload, the Attorney General can just say "We don't like you" and impose a censorship order on all US-based ISPs, ad services, and payment providers.
