The law at the time prohibited this because five years since her 18th 14th birthday had not passed when she gave birth to Dunham/Soetoro/Obama ...
I don't have the actual wording of the related statute to hand, but it stated that a mother had to be five years past the age of 14 to legally convey US citizenship to a child born outside the territorial jurisdiction of the US. Stanley Ann Dunham was just shy of her 19th birthday when Barry was born.
Thank you. Whoops. I were wrong.