Same reason a lot of other states still don't. Tradition.
Something people don't exactly reminisce about all that fondly or commonly is that back in the Little House on the Prairie days, actually well into the beginning of the 20th century, marrying what we consider underage was pretty routine, and I'm talking about preteens even. Especially in the South and rural areas.
People here talk about the Middle East and they're right, they are behind, by a mere century.