I can think of two reasons:
1) Kerry was convinced he was going to win and was trying to get a head start on the 2008 reelection campaign. He probably felt he had overspent and no amount of additional spending was going to change the outcome.
2) Kerry was convinced he was going to lose and was trying to conserve cash in a losing effort. No sense in throwing good money away after bad. He probably felt he had overspent and no amount of additional spending was going to change the outcome.