Not because of anything the pro-life presidents did. Because they didn’t do anything. They couldn’t.
There are two primary reasons abortion is declining - one good, one not so good.
The good reason is that more people are being educated about the nature of the life in the womb, and are realizing it’s not just a clump of cells. Public education campaigns and appeals to people’s better nature are working.
The bad reason is the disappearance of all shame and stigma attached to out-of-wedlock pregnancies, especially teen pregnancies. So many teens are getting pregnant now, it is no longer a big deal, and they don’t fear their parents or others finding out - assuming they even live with one or both parents.
It’s not because of anything any public official has done.