Nothing says he can't, except your assumption.
Then why does Obama still have a low-res version of the admitted forgery up on his "stop the smears" website as "proof" of his citizenship?
You're using a similar logical fallacy to those who said: "Why won't President Bush meet with Cindy Sheehan if he has nothing to hide?"
Or look at it from a game-theory perspective: if he doesn't have a legit BC, then he won't show it; if he does have a legit BC, why show it?
In fact, if he's legit, it serves him well not to show it except under his timing...perhaps after critics put themselves out there in a vulnerable spot.
Fact is, he's taking NO real heat from this, so why respond now and draw attention to it? A smart player doesn't let his opponent dictate his moves.