I'd quibble with you here that there actually are an alarming number of antisemitic nazis and xenophobic islamophobes in the US, primarily among Christians. Criticisms of all heinous religious practices will quickly devolve into bigots chiming in to complain about their personal ignorance. There is so much overlap between various religious beliefs and practices that practically any criticism of faith is valid against just about all religions. So most of the time, if you are singling out one particular sect or ethnicity, it's because of prejudice against those people.

Targeting Christianity ensures that you're not singling out one particular ethnicity or nationality, and it's far less likely to attract bigoted sympathies.