Because fools are dominating his party. One persistent vein of foolishness involves the ridiculous claim that Obama wasn't born in the United States or that he was born in the United States, but his original birth certificate contains some politically embarrassing, secret information.
So, Obama posted a copy of his birth certificate on his website. But we know how it goes with Rovian politics and the wingnuts who eagerly embrace every crazy allegation. They claim that the birth certificate Obama posted is a fake. They insist on a certified copy of his birth certificate or his original birth certificate. Ah, but what is to stop Obama from posting a fake certified birth certificate or a forged original? Of course, they must inspect his birth certificate in person. And who should get to inspect it? Why, every last skeptic in America. After all, why would anyone believe it if they haven't seen it with their own eyes. But even if every wingnut in America could inspect a certified copy of Obama's birth certificate with their own eyes, what is to stop Obama from showing them a fake "certified copy." On and on it would go. Folks, this is what clinical paranoia looks like.
Beyond naming the insanity in play, the political significance of the birth certificate story is that it is part of the wider Republican campaign to turn the political opposition into the big bad Other. As a political strategy, this effort seems to have backfired badly. Not only have Republicans tried to Otherize Obama, they've tried to Otherize vast swathes of America, declaring that only certain people they deem acceptable are real Americans. But it seems that they've 'Otherized' so many people, they may have effectively Otherized themselves in the eyes of a majority of voters who want sobriety, not paranoia.