Because they are lunatic Christians who think they are right and everybody else is wrong. That's why so many Christians become atheists it's because their people are shitty people who disrespect everyone else's belief. The cowards on the Internet.
Another reason why you cannot have a civil discussion with muslims: they make statements that contradict the statement they just made. They offer no evidence or proof there statements have any factual basis. They call names.
It would be wonderful to find muslims that would actually discuss islamic "discrepencies" with a scrap of integrity. All muslims will start out with how great their faith is, as long as they are asked no questions, they will remain pleasant. Once questioned about things that need some serious explanation that muslims do, they retreat, they get arrogant, they call names, they lie, they deny recorded events, they shut down, and then they will immediately return to the calm, pleasant person and tell you that islam is a religion of "peace".