A sizable chunk of Republican voters are adamantly against abortion, viewing it as murder and thus are unwilling to compromise on it. These people often support other Republican policies, but for many abortion is the ultimate issue. Some of them even oppose many Republican policies (I know a bunch of Catholics. Plenty of anticapitalist Catholics or one’s who support stronger social programs, vote Republican because of abortion. This demographic is why Louisiana gets some anti-abortion Democrats in office.)
To gain moderate voters, they have to alienate the very motivated anti-abortion voters. Alienating a sizable chunk of your voter base many of whom are reliable voters who vote in all elections, to gain new moderate voters, is a risky move.