One of the reasons is that the Republican Party has absolutely ZERO desire to win on the abortion issue. And that's because abortion gives the GOP a clear focal point with potentially unlimited organizing power. And it's an even simpler message to sell than religion, since we are "pro-life." (if that was true, however, they wouldn't be actively trying to repeal healthcare for up to 30 million Americans, nor would they be so pro-gun, pro-war, pro-death penalty, pro welfare cuts, pro- social security cuts, pro- drone strikes, etc).
The Republican party officially became "pro-life" in 1976, thanks to Jesse Helms (R-NC). The only reason no serious challenge was brought within the party against its move to pro-life was because, at the time, the Republican party was divided about the passage of the Equal Rights Amendment, "which was already under siege from prominent party members like Phyllis Shafly." If it had not been so busy attacking the ERA - with such dubious claims as it would "deny women's right to be supported by her husband" and it would "result in women being drafted into combat" - the Republican party would never have become pro-life."
Also, remember that Nixon had been seen as sabotaging Johnson's peace plan in Vietnam. And Republicans were ramping up the "Southern strategy" (which, while said to be an attempt to pander to racism in the south, was also an attempt to capitalize on deep seeded religious beliefs that rejected the "sexual revolution" and many of the ideas being advanced though feminism and "women's lib"), and trying to distance itself from the political hot potato that ended up in its lap of 60K dead U.S. soldiers in Vietnam. Since then, it has provided the perfect sheepskin to hide Reagan's contributions to the genocide of indigenous people in Guatemala (1981-1986) and numerous others acts of murder in Central America, including the assassination of Archbishop Romero in El Salvador.
It has also proved to be an effective way of convincing Conservatives that "Democrats" are no different than the German Huns a century ago, who, in order to drum up hatred of all things German among Americans, were falsely reported in countless newspaper articles of "throwing babies in the air and catching them on their bayonets."