Because the Christian Right has lost most of the battles in the culture wars and for a while, gay marriage was one they still had a fighting chance in. Even though they have now also lost that one, they still fight against the mainstream societal acceptance of homosexuality (exemplified most strongly in the legalization of same-sex marriage). Therefore, for them, it is one of the most obvious examples of society’s degradation. If they believe that God punishes society with natural disasters and pestilence for being corrupt and morally bankrupt, homosexuality is one of the clearest culprits since it’s the most recently accepted sin and they can’t help but see it as the reason for God’s ire. There’s a joke that the Christian Right is obsessed with the “three G’s”: guns, God, and gays.
To use a favorite expression of the alt-right net: we gays live “rent free” in their minds.