Fetal abnormalities are actually very common. There is a huge range of conditions that can cause a fetus to be born dead, die soon after, or live a severely limited life. In the UK, these are usually diagnosed in the routine scan that takes place at 20 weeks. Having an extra 4 weeks to make a decision before the legal time limit is really important for parents.
Here are some other possible reasons:
1. You didn't realize you were pregnant. This is more possible than you might think. Many women continue to have bleeds during pregnancy. Others do not have periods normally because they are on a form of hormonal contraception that stops periods, or they have irregular periods due to age (teenager or approaching menopause), unusually low or high body fat percentage, stress, or other conditions.
2. It takes you several months to raise the money for abortion. This is particularly an issue in the US, where abortion can cost hundreds or even thousands of dollars and isn't usually covered by insurance. There are millions of Americans earning just enough to cover their basic expenses. Suddenly raising thousands of dollars is simply impossible for these people. Recent laws requiring mandatory ultrasounds and counselling before abortion (which the woman must pay for) that have been introduced in many states have made this problem worst.
3. It takes you a long time to make arrangements for the abortion. Again, this is a problem in the US, where some states only have one clinic for the entire state (e.g. North Dakota, which is a larger area than the whole of the UK, has only one abortion clinic). It also has laws requiring an ultrasound, waiting period, and counselling, so a woman would need to make multiple trips to the clinic, each one requiring a drive of several hours. It can take time to book time off work and organize childcare for existing children.
4. Teenagers, recent immigrants with limited English, drug/alcohol users, women with mental illnesses, and women in abusive relationships all have trouble accessing abortion because they find it difficult to see a doctor and navigate the healthcare system. Teenagers may also take a long time to recognize symptoms of pregnancy.
5. In the UK, some women have needed late-term abortions because their GPs have lied to them and told them that they wouldn't be allowed an abortion. Most GPs are very good about referring people to abortion clinics, but some will deliberately delay the process. There are also waiting lists of about 3 weeks for NHS abortions in many regions.
BPAS published a list of reasons why women had abortions at 22-24 weeks at one of their clinics. It's eye-opening reading.
http://www.bpas.org/js/filemanager/files/bpas_press_briefing_late_abortion.pdf