There are two distinct reasons for this occurrence.
First, on August 28, 2008, Barack Obama made an exceptional acceptance speech. Even the harshest of critics had a hard time attacking it. In order to eliminate the positive coverage and impact of the speech, McCain had to change the media coverage cycle. The way to do this was to announce his Vice Presidential Nominee the following day.
Secondly, McCain celebrated his 72nd birthday on August 29th. It went without any fanfare and barely mentioned in the media. His campaign did not want to bring additional attention to his age.
Because he is a 4-time cancer survivor who just turned 72, his choice for the #2 spot is now a major factor because of his age. Although no one anticipates the worse for Senator McCain, if he becomes president and an unforeseen fatal event occurs, Sarah Palin would be president. Is Palin ready to lead?