I do know why. Things deteriorated when the state hijacked the church after Constantine.
Wherever the church has not been an established, state church, its history has been relatively honorable. Wherever the church has been an established, state church, you will inevitably find something dishonorable in its history.
There is nothing in the N.T. that presumes or dictates that there be established state churches. Indeed, the premise seems to be that the state is hostile toward the church, which it often has been. The expectation in the N.T. is that Christians will be good, productive, law-abiding citizens, and if they happen to be called to government service, that they will conduct themselves justly and honorably, but that the church as an institution and the state as an institution will have no formal relationship with one another.
In contrast, the Quran does indeed expect a fusion of mosque and state, and considers anything less than the establishment of Islam as the official state religion to be unacceptable. The Quran also makes it very clear that the employment of any means, including dishonesty, theft, slavery, and murder, are justifiable means for the end of advancing Islam.
True, some people who have called themselves "Christians" have conducted themselves just as badly at times in the past. If you study the history carefully, you will find in almost every case that an established state church was involved, and that the people behaving badly either had political/economic motives for doing so, or were ignorant dupes manipulated and misled by those who had such motives.
There is nothing INHERENT in the Christian faith that would lead a believer to engage in such evil, dishonorable conduct. There is very much something inherent in the establishment of a state religion of ANY variety that would lead the adherants of that religion to evil conduct that is actually in disobedience of the teachings of that religion. Except in the case of Islam, the bad behavior isn't even forbidden, but rather encouraged.
Which is exactly what is wrong in the Islamic countries. Their "holy men" are also Politicians, men of secular power, like Cardinal Richelieu (sic) and scores of others. It's not that way in Turkey, inspite of the efforts of the more fundamentalist among them, and they get along with the rest of world, save the Greeks pretty well (The antipath for the Greeks is more historical than religious, although there was a religous component too of course.). Religion can guide or influence the State, when it becomes the State, head for the hills, or the caves as in Heinlien's "If this goes On".