I'm gonna go out on a limb and say it's because of the acceptance of Christianity in general as being the norm in the US.
Most Christians, to this day, are very reticent to admit that a large number of Christians are 1. authoritarian 2. racist and 3. generally not good people.
This is because they've been using "Christianity" as shorthand for "good person" for decades instead of choosing to decide if someone is good based on their actions. They have a really hard time accepting criticism of their religion, even if it's totally valid. Even if they themselves could be classified as "good Christians."
From personal experience, my mother is one of these. She is not racist, she raised me with focus on the importance of learning and science, she has never judged me for becoming an atheist. However, she has a really hard time facing criticism about other Christians who are obviously deeply evil fucking people.
A lot of Christians got way too comfortable using it as a shorthand for figuring out who to trust in life, and have a "sunk cost" situation with that, and have a hard time letting go of the idea that Christian == Good.