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?
McCain, Cruz
No one asked McCain to prove he was an American.
The legal question was whether or not an American born in Panama was a natural born citizen.