Because he's the only candidate in recent history who was pressured to show his. Even though he has his short form birth certificate proving his birth, the state of Hawaii backing it up, it apparently wasn't enough.
Not to mention the references to Kenya and the claim that he is Muslim.
When was the last time a candidate had to prove he was American?
Any time someone disagrees with or holds Obama accountable, then that person is a racistt.
What was Trump disagreeing with President Obama on when Trump accused President Obama of having a fake Birth Certificate?