Partly, but mostly because he wanted to contrast with Bush (and Hillary to some extent) in order to get elected.
Whether he was a true believer or not, it's a lot easier to say you're going to be transparent than it is to be transparent or perhaps better stated, force everybody in the Executive Branch to be transparent.
Hell, I'm sure I'm less transparent as a professor than I thought I'd be as an optimistic grad student 11 years ago. POTUS? $10 says he walked into the White House on Day 1 and somebody who has been doing this a long time told him, "Yeah, Mr. President, we're not going to be able to do that and here is why." I gotta think that first day when the President learns where all the bodies are buried is one of the biggest shit your pants moments a person could have.
W got mocked for saying this in his first debate with Kerry, but I never forgot it. Being president is hard work.