The main reason to withhold an actual birth certificate is because he cannot produce it. His actions (or lack thereof) are driving the conspiracy theories. In this case, the evidence seems to indicate that Americans may have elected the first illegitimate president. The social security number issue and the stories about BO traveling on an Indonesian passport in his younger days just add icing to the cake.
He basically tried to do that when he decided to run for the presidency. It looks like the Certificate of Live Birth (not a legal Birth Certificate) that he displayed on his web site was applied for in 2007. His sister also has a Hawaiian Certificate of Live Birth, even though she was born in Indonesia.
Originally Posted by m42
"Democracy is two wolves and a lamb voting on what to have for lunch. Liberty is a well-armed lamb contesting the vote." -- Benjamin Franklin