Obama got more of the youth vote, Hispanic, Black, and women vote. These facts may explain this somewhat:
Who's having abortions (age)?
52% of women obtaining abortions in the U.S. are younger than 25: Women aged 20-24 obtain 32% of all abortions; Teenagers obtain 20% and girls under 15 account for 1.2%.
Who's having abortions (race)?
While white women obtain 60% of all abortions, their abortion rate is well below that of minority women. Black women are more than 3 times as likely as white women to have an abortion, and Hispanic women are roughly 2 times as likely.