Because at this point, we only know what we are allowed to know. Facts will continue to surface for the next three decades.
Also we have no way of knowing the future. There is no way to tell if events will play out which will vindicate or condemn Bush's actions and inactions.
With that in mind and with Bush's second term winding down, enough facts have surfaced to lend more weight to the negatives rather than the positives.
But, things can change drastically with just a few revelations.
It was 30 years before we found out about JFK and Marilyn Monroe. Compare that to the glowing LIFE magazine accounts of life in the holsom, ever-faithful Kennedy White House when he was alive.