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.

[1] https://books.google.com/books?id=cf5RFWIMJugC&pg=PA42&lpg=P...

[2] https://en.wikipedia.org/wiki/Francis_Schaeffer

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.

I’ve answered elsewhere, but note that conservativism isn’t one and the same as the Republican party; nor is it the same as the evangelical crowd.

But tl;dr; answer: Modern abortion has its roots with a founder who was also strong advocate for eugenics.

She was a product of her time and really believed she was advocating for improvement of society, but the ideas took a dark turn.

The only notable opposition were from the conservative religious circles; but as noted even there the record is spotty.

Still, credit where credit is due.

Whether more modern evangelicals are rooted from the same opposition is irrelevant.