The campaign isn’t saying and the Politico is speculating about the delay:

He has collected millions of dollars. Aides go to work every day in a secret suite of offices in McLean, Va. He has policy briefings. He is preparing for nationally televised Republican debates. He just named a communications director and a political director.
But yet, Fred Thompson still says he’s not sure he’s going to run for president.
The “Law & Order” actor and former U.S. senator from Tennessee is delaying his formal entry into the race in part because his current status makes him less of a target and lets other candidates continue to get chewed up. “We’re in no hurry,” one close adviser said mischievously.
But there’s another big reason. If he were to formalize his candidacy beyond its current “testing the waters” status, Friends of Fred Thompson Inc. would have to reveal its contributions and expenses this week.
Thompson’s aides say they will not file such a report, and the Democratic National Committee plans to argue aggressively in coming days that he should. Democratic aides argue that Thompson could be violating campaign finance laws by “ducking” a full filing.
Filing as a candidate also triggers other legal requirements, including personal financial disclosure.
[…]
Another Thompson official said: “When you’re trying to determine whether you can be a viable candidate for president, you’re talking about a nationwide fundraising effort. The FEC rules recognize that the ability to raise money is an important factor in determining whether a person is a viable candidate for president, just like traveling the country and collecting endorsements. They’re all activities to gauge your potential political strength.”
[…]
Thompson advisers said the former senator does not plan to disclose details of campaign finances to the FEC until the report due date of Oct. 15, although the campaign may make an announcement about its very successful fundraising long before that, probably in conjunction with a declaration of his candidacy.
Senior Thompson advisers had talked about announcing the week of the Fourth of July, then sometime in July, then the first week in August. On Friday, one top official even mentioned September. Thompson’s team says it’s up to the former senator, and they really don’t know. The point, they say, is that he’s going to wait as long as possible without deflating the enthusiasm of his many vocal grass-roots supporters.
[…]
Members of the campaign continue to discuss whether Thompson should participate in a debate on Aug. 5 in Des Moines, Iowa, sponsored by ABC News and moderated by George Stephanopoulos. One member of the Thompson team had told The Politico early this month that the former senator was beginning to prepare for it. And some of the announcement-date conversations had taken that debate into account.
But now it sounds like Thompson may not participate, although his advisers will not give a definitive account of their thinking. One factor his team is discussing is how much criticism he would take for avoiding questioning by Stephanopoulos.

I think he should seriously think about making the August debate so that he can put to rest the rumors concerning his position on abortion. The speculation needs to be answered once and for all in a public forum so that it can be taken off the table as an issue. If I were him I would say that my record speaks for itself and I’ve stated very clearly that I support the ban on federal funds being used for embroynic stem cell research which is about as pro-life as a candidate can get (which is a little dig at the rest of the field) and as a federalist you know that I will be selecting strict constructionist judges. Then if it becomes an issue later on in the campaign, he can always refer back to his statement (asked and answered, let’s move on).