Because there's cases where abortion can be justified, on a case by case basis. You can't and shouldn't generalize every abortion case with the same brush. To say all abortion should be stopped is wrong. If a woman gets raped or is a victim of incest, she shouldn't be forced to keep that child if she doesn't want to.
If you think it's morally OK to terminate a pregnancy that was a result of rape, then why isn't it morally OK to terminate a pregnancy in any other situation? The woman's situation is different but the foetus's isn't - I thought the pro-life stance was based on the right to life of the foetus?