The first paragraph of the latest AP article sums it up:

A newly disclosed effort to keep Vice President Dick Cheney's visitor records secret is the latest White House push to make sure the public does not learn who has been meeting with top officials in the Bush administration.

The Bush administration has changed the rules over record-keeping to prevent access to visitor records.  They've usurped the role of the Secret Service, which is covered by the Freedom of Information Act (FOIA).  Instead, Bush and Cheney claim visitor records are under their jurisdiction, thus exempt from FOIA.  CREW is not accepting that determination, hence our lawsuits.   Also, as AP notes, this controversy took off following the Jack Abramoff scandal:  

The administration is seeking dismissal of two lawsuits by liberal watchdog group Citizens for Responsibility and Ethics in Washington that demand Secret Service visitor logs.

In trying to get the cases thrown out, the Justice Department has filed documents in court outlining a behind-the-scenes debate over whether Secret Service records are subject to public disclosure. The discussions date back at least to the administration of President Bush's father and involve the Justice Department and the National Archives as well as the White House and the Secret Service.

The government's court filings show that the Bush White House focused on the issue in the months before Election Day 2004.

Discussions moved into high gear when the Jack Abramoff lobbying scandal prompted news organizations and private groups to demand that the administration turn over Secret Service records of visitors to the White House complex and the vice president's residence.

Why is the Bush administration hiding visitors?   Clearly, they've got something to hide.