Politics. Having produced what he says is his COLB, Obama believes his proved his citizenship. He won't respond to any further demands for documentation because no other presidential candidate was forced to do so. To do something that no other presidential candidate was forced to do would be an admission that his COLB wasn't sufficient.
Give me a reason that would supersede the U. S. Constitution.
Show me where in the Constitution it says that a candidate must prove his citizenship and who he must prove it to.