Because he cannot. Revealing his past would destroy him. Not revealing his past will eventually destroy him.
His past will literally devour him at some point.
We should relentlessly demand that he show us evidence that he is eligible to be president. He can not. So we continue to call him on it, without letup. And that includes all the people trying to get the Republican nomination. Anyone who has a soapbox and remains silent is guilty.
Relevant: If anyone has a link to a statement by anyone credible and in a position of public notoriety ever asking, publicly and directly whether "Obama" is constitutionally qualified to hold office and been answered in the affirmative, said answer being straightforward and public as well, I'd very much appreciate their posting it. I personally cannot recall ever seeing such.
"His past will literally devour him at some point."
From your keyboard to God's inbox.