Two good reasons:
1. His father was a British citizen, thus obama cannot fulfill the requirement for a Natural Born citizen.
2. If he was born in Kenya as his relastives state, his mother was not old enough to convey citizenship to him as per the statute in effect at the time.
Also, no evidence of a hospital birth certificate has ever been found in Hawaii.
Yes, you can argue that Barry is not a natural born citizen, but you do not have to be a natural born citizen to qualify for POTUS.
There is nothing in the Constitution requiring that the mother be 18. That comes from a misreading of Hawaii law.