Maybe he was afraid that they would find out about his crazy taste in art?
It's a good question. Maybe it was just another way for him to get off on his power trip, or perhaps he knew that if the mass graves were found while he was in power that he would lose whatever aid or support that he had from other nations.
It will be interesting to see if Bush & Co. find any WMD's -- so far it isn't looking good for them. Anything is possible, though.