Potential reasons:
1) because the original long form document is from a Nation other than the United States.
2) the original long form states he is of parentage other than what his CURRENT short form says.
Whatever it is, it's obviously a deal-breaker for either cultural, or legal reasons. My guess is that he lied about his provenance in interviews, which forced his campaign staff to manufacture a forged COLB that matched his statements. Of course the original says different.
He's already not constitutionally qualified to hold the office of President, per a strict reading of the NBC clause in the Constitution, but the fact that his (alleged) father was a Kenyan is public knowledge.
He's obviously not trying to hide that, so what else might he be trying to hide?
I would vote for place of birth being Kenya, and not Hawaii, but that's just my personal speculation. Who knows what he's hiding?
My guess is he is using it as a smoke screen. Hiding the whole issue behind something he and Dims can mock... ie: birthers. What it’s hiding is in plain view. He was British at Birth. You cannot hold two citizenships and be a Natural Born Citizen at birth. In fact, because of his heritage, Obama could never have been a Natural Born Citizen even if he was born in the Oval Office.
That is what its hiding in my opinion. He was listed as British under his fathers name, under his fathers heritage.