There are a myriad of reasons. His mother's whereabouts at the time of birth are not documented. His step-grandmother says he is a son of Kenyan soil. His own wife says Kenya is his home country. Tammy Duckworth says he was born in Indonesia. Obama gave conflicting accounts about which hospital he was allegedly born in. The state of HI refuses to confirm that his alleged COLB is real. He has a certificate number that is most likely fraudulent. Hawaiian birth certificates are easily obtainable for people born out of state. What we have is little reason to conclude that Obama was born in the U.S. with an American mother. The evidence is sparse and uncompelling.
Since you ducked out of our earlier conversation, perhaps you missed where I pointed out that both of these accusations are incorrect. Tammy Duckworth did not say Obama was born in Indonesia; the author of the article himself has stated that that was his error, not hers. And Obama has not reported that he was born in two different hospitals. Obama has consistently stated that he was born at Kapiolani. I'm at a loss where you even got this idea.
But since you keep repeating the latter claim as factual, despite being informed that there have never been any such statements as you claim, I invite you to provide links to any of these supposed "conflicting accounts" where Obama named some other hospital as the place of his birth.