Because Obama's own books, whether he wrote them or not, say his father was a British/Kenyan citizen. We also know that BHO Sr came to Hawaii in '59 and left in '62, and left the US, IIRC, in '64. By some accounts, which I find credible, the Natural born citizen requirement means more than merely born in the US. So the BC would prove who his father was, thus proving or disproving his ineligibility. When he sends legions of lawyers out to prevent the release of his early records, including his Birth Certificate, that leads somewhat naturally to the question of "just where was BHO Jr. born". For other Presidents, we have signs, national monuments, etc. For BHO, we don't even have a plaque at the entrance to the hospital, if any, in Hawaii that he was born in. Now why would *anyone* not want to say what hospital they were born in?
Other Presidents and candidates did not come to the campaign with virtually no national recognition, beyond a speech at their party's convention 4 years (actually less than 3 year from the day he announced). McCain we knew about, his father and grandfather were Navy Admirals. His picture is the Annapolis yearbook, and we saw him getting off the C-141 from Hanoi in February of '73. We know his ties to the US.
We know Biden is one of the youngest persons ever elected to the Senate (He was 29 on election day 1972, but turned 30 before taking office, as the Constitution requires). We knew he was an idiot and plagerist, but those are not, unfortunately, Constitutional disqualifications for either Senator or Vice President.
Obama has been President for less than nine months. What Presidents, other than maybe George Washington, have had signs or national monuments erected in their honor during their first nine months in office? Wouldn't that be pompous as heck?
I can also confirm for you that the hospital that George W. Bush was born at doesn't have a plaque saying he was born there. Reportedly, the most they've done since 2000 is mention his birth there in a hospital visitor's brochure.
Now why would *anyone* not want to say what hospital they were born in?
He has said. It was Kapi'olani Medical Center. He wrote a letter saying so. He's never said he was born anywhere else. In eight years in office, did George W. Bush ever publicly state what hospital *he* was born in?