They haven't been, in the long view. Historically abortion was a Catholic issue, and sometimes liberal Protestant[1]. It generally wasn't an issue for Evangelicals, who were also mainly on the political left. Then in the late 60s the teenage son of a popular Evangelical pastor/author[2] became concerned for the rights of the unborn, convinced his father to take that message to the masses, it worked, and Republicans figured out that it would be a way to build a new coalition.
If you look at the extreme countercultures of the 60s, you see one side who hated "the system" that produced mass affluence, but loved the fruits of it. The other side loved the system, but hated the fruits. So one way to look at it is as a sort of political shibboleth indicating that you were definitely on one side and not the other.
US evangelicals had a mix of indifference or outright support. However it’s a mistake to confuse “conservatism” with religious organizations and even a party.
That it really took off in the 1960s+ isn’t surprising with Nazi death camps fresh in everyone’s mind, leaving a very bad impression with anything tied to eugenics and its proponents.
While I don’t agree that abortion is equivalent to eugenics—-sorry, if my wife were to die otherwise I’d choose abortion—much of the revulsion has its moral roots in the argument against eugenics.
They haven't. It's a political alliance with the anti-abortion crowd. The 1976 Republican party platform was the first to include a stance on abortion.
