There are lots of reasons. For one, Christians believe that the life inside a womb is a human life, and there is no good reason to take an innocent life. Most believe that since all the genetic information needed is there at conception, and since all that needs to be added is time and growth, that the creation is a person. Christians believe that God has made each of us, starting in the womb (Ps. 139: 13-16*) and that if started, we each deserve to be carried to term unless there is a direct and potentially mortal assault on the life of the mother (that’s considered self-defense and therefore justifiable).
There is a deeper concern as well, and one that is harder to understand.
Many in society think of abortion as either a private medical issue or a
social issue. Christians don¹t accept that just because some folks want to
define the issue as a private matter, that it is simply that. Christians
also don¹t see it as just a social issue. We view this as a moral issue that
ultimately demands a judgment from God. If the figure of 60 million
abortions since Roe v. Wade is correct, then we have the guilt of 60
million killings hanging over our head. For those of us that believe in a
God of justice, that¹s a scary situation, and explains a great deal of the
angst, heartbreak, and occasional anger that Christians feel.
But isn’t legal abortion the law of the land?
The fact that abortion is the current “law of the land” holds about as much
water to most of us as the fact that slavery was legal for hundreds of years
and that the Dred Scott decision was also “the law of the land.” Yes, we see a correlation there, and believe that those who support abortion are on the same wrong side of history as those defending slavery. We also see a
connection among America’s “Three-Fifths Compromise,” the dehumanizing of the Jew during the Third Reich, and the current attitude toward life in the womb. To Christians, the thinking is essentially the same, and is to be opposed for the same reasons.
Like the “private matter” perspective, the “women’s rights” perspective is
not one that carries the day with Christians, either. In fact, Christians
lament the fact that the many good advances made in the area of women¹s
rights have been folded in with the issue of abortion, to the point that
abortion is the obvious centerpiece of the discussion. Christians are not
against women voting, respect for women, or the female orgasm. We just don’t equate free and/or easy abortions with freedom for women; in fact, we
consider women “the other victims” in an abortion, and believe that science
will continue to show that abortion has grave psychological and some
physical effects for the woman who has one.
Everyone devalued in our society, and sensitive and observant souls rightly
lament that. Christians feel that a great deal of that devaluing comes from the acceptance of the taking of life in the womb.
There are many other facets to the issue, of course, but these are the main
reasons why Christians oppose abortion and why the issue won’t simply “go away.”
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* Psalm 139:13-16
For You formed my inward parts;
You covered me in my mother’s womb.
I will praise You, for I am fearfully and wonderfully made;
Marvelous are Your works,
And that my soul knows very well.
My frame was not hidden from You,
When I was made in secret,
And skillfully wrought in the lowest parts of the earth.
Your eyes saw my substance, being yet unformed.
And in Your book they all were written,
The days fashioned for me,
When as yet there were none of them.
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For the film folks among us, check out a related (and somewhat long) article “Abortion and the Film Image” at http://filmprof.wordpress.com/2011/09/.
Mark DuPré 
Pro-choice advocates are fond of accusing pro-life people of wanting to control women’s bodies. But of course that entirely misses the point of why Christians are anti-choice concerning abortion. We have no interest in dictating what women do with their own bodies. We have a great interest in protecting that separate human life that is being shaped within a mother’s womb.
The analogy to slavery is apt. I say this as an African-American who, as a Civil War buff, has some knowledge of what went on in that period. Abortion represents that same pattern of the powerful exercising arbitrary and unlimited control over the weak for their own profit and convenience.
Perhaps if pro-choice people could at least understand and acknowledge the real reasons Christians are against abortion choice, it would bring more light and less heat into the debate.