In all likelihood, [film director] Nichols had a "revocable living trust," that contained the dispositive provisions of his estate so that his wishes were shielded from the public.
Celebrities and high-profile estates tend to be a hot topic in the media, but sometimes the media doesn't get the full story.
An article from The National Review, titled "The Death of Mike Nichols and Estate Planning," sheds some light on why we know so little about the estate of award winning director Mike Nichols.