Because she was never realy married to 0bama, isn’t the mother of Zero, and needed to get the name “Obama” so she could have custody of Zero, and create the legal fiction that the Kenyan and she were his parents and that Zero’s name was BHO2.
And don’t forget, that name change was handy for when she started at the U of HI in Spring 1963; it allows the university to issue a letter stating that Stanley Ann Dunham enrolled at the university for FALL 1960, when Stanley Ann Dunham wasn’t in Hawaii, but Ann S Obama was.
Tricky.