It is hard to believe that Christian monks could produce benign histories of Islam since many of them lived during the times of the Muslim invasions of Europe and knew how brutal the Muslims could be. There were scholars and monks all through the middle ages, even the dark ages. It is not as if later scholars had to look up Muslim histories to know what went on in those days. My hunch is that probably the anti-Christian biases of the age of reason began to show the Christian world as evil and backward while the *other*—the Islamic world had to be portrayed as a paradise, comparable to the ‘noble savage myth. Anything that was not European had to be better than European culture. This anti-west attitude has grown with time and is now a huge part of western thinking on the left. I am just guessing but it wouldn’t suprise me if this glamorizing of Islam did not begin at about the time of the enlightenment.
The Christian and Jewish scribes were among the few educated, multi-lingual/literate survivors of Islamic conquest whose skills enabled their survival under intensely humiliating circumstances. Many societies disintegrated under such pressure.