I believe as it has been pointed out several times. W has recently had back surgery. He may not be able to travel either. Although the Mrs. Bush could/should attend.
Oh wait...I take back what I said.
These sources say Bushs doctors told him to give up golf until his back healed, but he managed to sneak in some chipping and putting in Florida and has now been seen back on the links at a Dallas country club.
Of course isn’t there still a pesky arrest potentially pending if he leaves the country for war crimes?
If, however, his back is the cause, then he should have publicly stated that he wanted to attend, and that's the only reason that he's not going..because of his back.
But that would have made Carter and Clinton look bad...