I'm going with Arthur's answer. To answer the question of why the same doesn't apply to Christianity, I dunno, maybe because Christianity is a lot more widespread than just one area of the globe. Maybe because criticism of Christianity is less popular/ not a cultural phenomenon, more of an individual-here-and-there thing. Maybe because the shortcomings of Christianity are less stark. Maybe because Christianity is older and more globally entrenched so it's taken more as an Earthly given.