Probably because of BHO Sr.'s polygamy problem.
BHO, Sr, told his university he had divorced Kezia in 1959, before he left Kenya. In 1964/5 he married Ruth Nidesand in Kenya. The marriage was statutory, ie monogamy.
Bigamy has been a crime in Kenya since 1902.
He wasn’t arrested for bigamy and Ruth didn’t cite bigamy as grounds when she divorced him. She cited cruelty.
His family say he divorced Kezia in 1959.
Looks like he might have been telling the truth, for once.
Anyway the INS couldn’t prove bigamy either, and they tried. The British wouldn’t confirm he’d ever been married at all.
If the British said he wasn’t married US law had to presume he wasn’t. So, the marriage was legal.
The big question would have been whether his marriage to Stanley Ann Dunham was bogus. IMHO it was. If the British had asked the INS for details and decided it was they’d have revoked his passport and he’d have been deported ASAP. The INS probably wanted to do just that.