Because liberal Cardinals like Cushing and Bernadin pushed the "I'm personally opposed, but" argument or the "Seamless Garment" idea that ALL social justice issues are equal, so if you're anti-nuke, or anti-death penalty, or are concerned for the poor, you don't need to worry about whether a politican supports abortion, because those other things are just as important. And besides, Republican hate poor people, or so they've been brainwashed to believe.
Very good point!!