Because the pregnancy is a result of her irresponsibility not the actions of some other person. You know when you have sex that pregnancy is a possibility and you should be prepared for the outcomes of your actions before you do them. You shouldn't put yourself in a position where X might happen if you are not prepared for X to happen. Now with sex, people do it anyway even if they aren't prepared. This is irresponsible. Having abortions legal takes some of the responsibility off of the situation because the problem can be fixed, but without abortions the responsibility is placed back on and it should be placed back on the individual.
As many of us have long known the anti-abortion movement has nothing to do with saving lives or anything of the sort. We can clearly see that it is nothing more than an attempt to enforce one set of sexual morals on society.... "irresponsible sex" etc. If you don't like abortion don't have one.
As EZscience has stated; the world is already overpoputed. Not only do we need abortion we need MORE abortion.