That's because they haven't figured out how to play their cards right.
The fact is that the vast majority of the population favor tighter restrictions on abortion than exist now, but a largely-overlapping vast majority do not, today, favor restrictions as tight as what the most pro-life groups seek. Many of the people in the overlapping segment are, for example, strongly opposed to abortion restrictions on rape and incest victims, but favor restrictions on late-term and "partial-birth" abortions.
If the Republicans were smart, they would work to grab the votes of this very large segment of the population which is currently being courted, quite successfully, by the Democrats. Push hard for the restrictions that the vast majority of people want, while holding off on those that people don't yet want, and move forward from there.
Does anyone see any problem whatsoever with my strategy? Any idea why Republicans can't figure it out?