Oh this one is easy. Being pro-life is too close to be pro-God and you can't cross that fictional wall of separation according to libs. Once you make a judgment that life has higher value than government the libs lose their ideology. Government should be the highest value for a lib...THEN community...THEN individual concerns...(life, liberty, pursuit of happiness) If it ever gets that far.
By valuing life the de-value GOVERNMENT and this is strictly forbidden with the liberal play book.
There is also a smaller element that says young poor uneducated girls can't control their own lives (sex drives) and need the government to provide relief to them in the form of abortion to end the unwanted pregnancy..but the former is much more the case than the later.
Maybe it's not as "easy" as you think -- there are plenty of pro-choice conservatives, and pro-choice Jews/Christians.
Anyone who tries to dismiss it as just being an "atheist/liberal" thing is kidding themselves.