>Why.
I don't know if that's a question or an unenthusiastic interjection, but it's because Christianity is a religion for suckers. Those posters who rightly realize Christianity for the scam it is decided to publicly mock its adherents.
However, if you're asking why these threads haven't been deleted, then I don't know. Personally, I think all religious discussion should be removed from /pol/, because it's nothing but shitflinging. Political discussion that involves religious groups is fine, but discussions about the tenets of some religion are just a waste of time.