Because the fetus is a human being also.
This is close. If you go deeper you may see that it the real issue is that you not only want to be a good person but you are deeply committed to being one too and your understanding of abortion, your position on it can't be other than it is because your understanding of what the good is has been conditioned by a need to look at the world as black and white, that moral complexity must never be considered because it will threaten your fear of uncertainty. You might make a mistake and turn out to be actually evil.
For example, suppose a human life is worthless. Don't like that? How about masturbation is killing millions, maybe billions of unborn children? Would that more apply to you? If you want to be moral and morally wrong at the same time it is very important never to think. Never look at the implications that for your moral beliefs that are personal to you, you are more than happy to tell others what their moral values should be, that a woman, in order not to commit a crime in your mind and about which you maintain a powerful will never to examine, winds up meaning she should be forced to carry a fetus, maybe one born of rape.
The problem as I see it is that because everyone who has any moral sense at all knows that human life is, lets just use the word 'sacred', this becomes a moral absolute. But there are other moral absolutes and one of them is that you have no right to impose your moral absolutes on other people or you will have them apply theirs to you. Let he or she who is without sin cast the first stone.
When two absolute moral positions conflict it is up to liberals with their greater logic and greater ability not to need to protect their moral beliefs from clouding judgment to find rational compromise. It is the duty of a secular state to determine when abortion should be legal and when it should not, and that requires people who will not judge the situation based on religiously held beliefs. What conservatives have done to protect themselves from the fear of sin is throw secular society under the bus.
Repressed conservative fear always leads to the witch hunt and the Spanish inquisition, politicians cheating to stay in power because of the certainty that God is on their side.
It is not enough to want to be good. You also have not to know what the good really is because only blind bigots and dangerous fanatics think they do. Abortion does not threaten the good. It is a demonstration the competing goods require intelligent compromise.