Condoms break, on some people birth control pill doesn't work, antibiotics makes the pill not work, and condoms aren't 100% they are only 76% accurate. When you look at a condom there are tiny holes in it and sometimes sperm and other STDs can swim straight through it.
Fifty-four percent of women who have abortions had used a contraceptive method (usually the condom or the pill) during the month they became pregnant.
Forty-six percent of women who have abortions had not used a contraceptive method during the month they became pregnant. Of these women, 33% had perceived themselves to be at low risk for pregnancy, 32% had had concerns about contraceptive methods, 26% had had unexpected sex and 1% had been forced to have sex.
There are many factors that affect contraceptive use. First and foremost, we must remember that no form of contraception is 100% effective- they all have failure rates, and what's more, there are two different failure rates- those of perfect use (i.e. how many times the method fails when used perfectly in laboratory conditions) and those of practical usage (i.e. how many times a method fails when used by the average person, who may make a mistake like rolling the condom on the wrong way, or accidentally missing one her pills, etc.).
And of course it doesn't help those practical usage failure rates when we have abstinence-only education, which not only doesn't prepare teenagers to be able to have safe, protected sex that prevents pregnancy and STD transmission, but also has been shown to often be full of inaccuracies and misinformation, like exaggerating the failure rate of birth control. If a student reads in his or her abstinence-only textbook that condoms have a fairly high failure rate, he or she may think "Why bother using one at all?" in the heat of the moment. And let us not forget that studies have shown that students who take abstinence pledges are less likely to use protection when they do eventually have sex- probably because they believe that they will fulfill the pledges (despite evidence that abstinence fails anywhere between 26%-86% of the time) and therefore do not need to keep any protection with them.
Then we also have questions of access- when asked "Has there ever been a time in your life when the cost of prescription birth control made it difficult for you to consistently use it?", 34% of women responded "Yes", and of those 34%, 55% were young women and 48% were under the $40K income level. Women may also find that, thanks to conscience clause laws, pharmacists are able to deny fulfilling their prescriptions for birth control or emergency contraception.
There are also cases of reproductive coercion, where abusive partners may sabotage their partner's birth control in order to get them pregnant. And yes, there is rape as well. In terms of consensual sex, it also bears reminding that sometimes people want to get pregnant at first, and then if the situation changes, may change their minds- this also contributes to the number of abortions.
I've written a long blog post about how we can reduce the rates of unintended pregnancies (and hence abortions) here:
Well in our case it was just one of those things, he had a vasectomy and had been fine for years but they body has amazing powers of healing and that's what happened. Sometimes things just fail for example condoms have one of the worst failure rates for pregnancy and even the pill is not a guarantee. Every method of birth control has a failure rate with the exception of abstinence. So you can buy a condom and take the pill and still end up pregnant.