I don't know of this was "the reason," but turning abortion into the moral cause for right-wing evangelical types made the Republican Party and the right wing in general more palatable for Catholics, who traditionally in America had been more working class and left leaning. Besides abortion, Catholic values are big on helping the poor, providing health care and education, etc.
Catholics are also supposed to be against the death penalty. A lot of early labour movements had many Catholics (and Jews.) Lots of 60s and 70s activist nuns were out there supporting Civil Rights, opposing war, opposing nuclear weapons, opposing the death penalty, advocating for social programs for the poor, etc. But they were also all ostensibly pro-life.
But things like poverty, war, and racism are complex and hard to grabble with. The death penalty is bad, but people don't generally have sympathy for the actual convicts on death row, so that's more of a philosophical issue for most people. But if you truly believe that abortion is actually murder, of the most undeserving and innocent victims imaginable, then you'll be easily exploited by people with the same agenda. You can put those other issues on the back burner because babies are being murdered!
Even though I honestly despise the church, I tend to find myself defending Catholics online a lot, especially when people lump them in with creationists. A monk taught me evolution. A Catholic priest theorized the Big Bang. The pope gave permission for people to skip mass during the pandemic, and my parents' bishop actually FORBADE people over 65 from attending mass until it is safe. The Vatican also posted a pretty thorough report on the stem cells used for the covid vaccine and why it's okay for Catholics to receive it. They're a pretty pro-science bunch, for a group of people who think one man has a magic phone to God.
Forgive the sperg. This is all to say that Catholics in America tended to be very solidly left-leaning, traditionally. Abortion was the issue that made a good chunk of them turn right. So again, I don't know if that's THE reason Evangelicals suddenly became interested in abortion, but I think it increased their power as a voting bloc, and somebody must have noticed. (Also, as a tool to fight feminism. They realized more young women would leave their congregations if they weren't anchored down by marriage and babies.)
Of course most average Joe Catholics are a bit more pragmatist than to only vote on abortion-- they'll weigh it with their other values, or recognize that things like education help reduce the abortion rate. But there is a rising trend of conservative Catholics (the ones who said Pope Francis should be careful about calling for more taxes for the rich) and Trad Caths. They have all the crazy fundie nonsense with the grandiosity of Catholicism. It's gross.
