I think there's also a combination of factors at play. The first is that in American politics abortion, gay marriage and gender politics are more touchy, hot button issues than others like food stamps for the poor, climate change, or even immigration and gun control which can be hot button but still doesn't rise to the same level. So if a Bishop says they are against Marriage equality, that gets more media coverage than a bishop who stands with undocumented migrants. 

The second i think is the nature of Catholic social teaching. It doesn't fit neatly into liberal-conservative, left-right categories. It's conservative on reproductive issues, marriage, the family, gender, sexuality which are all the hot button issues in American culture. It's liberal on social justice for the poor, workers rights, the rights of immigrants, abolishing the death penalty, being anti war, etc. So it also depends on which issues you might prioritize in how you look at the Catholic bishops.