Why? Because Bush whose popularity was sliding amid rising questions about the war in Iraq — among other things — knew that there was no path to victory against Kerry by spending any substantial time touting his accomplishments during his first four years in office.
Partisans on both sides were already lined up either for or against Bush and no amount of positive (or negative) advertising would move them off of how they intended to vote. Undecided voters didn’t like Bush so positive ads amounted to a waste of time. The only way to win was to make Kerry even less palatable.
Obama is in a somewhat similar — albeit it slightly stronger — position that Bush found himself at this time in 2004. The struggling economy has dragged down the current incumbent’s numbers and two of his main legislative achievements — health care and the economic stimulus — are not popular with the American public. (They are popular with the Democratic base, however, which is why Obama is touting some of those accomplishments in web ads — a means of communication that helps gin up energy in the base.)
And, like Bush, Obama is a known commodity for voters while his opponents is not.