Nothing besides abstinence is 100 percent but using birth control correctly is very effective. I think the reason is because of poor communication with parents. They might feel neglected and want to have a baby to make up for the neglect or love that they don't get from their parents. I think it is lack of responsibility. A percentage of teens are going to have sex, but of those teens, not all use contraception carefully, so I think that inconsistent use of contraceptives is one of the main reasons.
Birth control can be hard to get, and sometimes expensive to get.
Being from a small town, I know just about everyone who works in the health department. I know I would never go in there to receive free condoms. I don't think that it is so much poor morals and values or birth control. They just don't care, or they think it won't happen to them. I think that teens have sex and get pregnant because they are pressured into it by friends, or they don't want to disappoint their boyfriend or girlfriend.