Hard to build a case and prosecute wrong doing by White House or any other government officer, intentional or otherwise, without access to evidence.
What better way to sweep the governments mistakes, misscues and corruption under the rug! From the Secrecy Blog:

The state secrets privilege has been invoked by the Bush Administration with greater frequency than ever before in American history in a wide range of lawsuits that the government says would threaten national security if allowed to proceed.

In virtually every case, the use of the privilege leads to dismissal of the lawsuit and forecloses the opportunity for an injured party to seek judicial relief.

Terrorists can kill people and destroy property. But they cannot undermine the rule of law, or deny injured parties access to the courts. Only the U.S. government can do that.

The state secrets privilege has been invoked lately in a remarkable diversity of lawsuits. See this selection of case files from recent state secrets cases.

POTUS sure must have a lot to hide considering how pervasive secrecy has become in the halls of our elected leaders in Washington!