Because Kenya was still part of the British Empire or Commonwealth at the time.
Eight years ago, I posted multiple examples of the same date formats from Kenya during that same time era.
Those would be exceptions, possibly fakes, but certainly outliers.
The fact is, 0bama had another BC because he wrote about it in his book.
Maybe, but the Kenyan birth story isn't very credible.
Apart from all that, consider: not producing a birth certificate early on didn't cost him a single vote, so there was no motivation for him to release it.
All the to-do about his being born in Kenya was something Obama used to make his critics look silly, so it paid off for him to withhold his documents for as long as he did.