Because they hate women, and wish to oppress and torment them. Forbidding abortion is just a useful club for them to beat on women with.
They don’t care at all about the “unborn”; their behavior simply does not fit their claimed motivation. They show no concern at all for the health of the fetus, and none for the baby they’ve forced to be born. Nor do they care if the fetus is doomed to die, or already dead, so long as they can force the woman to birth it. Nor in places where they can get away with it have they hesitated to condemn women to a slow death by forbidding them an abortion to save their life. Nor have they showed any great interest in actually doing anything that reduces the number of abortions; only in making them less safe and more degrading.
The so-called anti-abortion movement is a hate movement directed at women, nothing more. A more successful, gender targeted equivalent of the KKK.