The middle ages started after the fall of the western Roman empire.

So there was more exposure to different races that what's conventionally thought by people in the time periods.

Like a across the Mediterranean from Greece and Italy is Africa.

Conquering Northern Africa have been a goal for centuries before that.

Also the middle East has also been a goal to capture.

So for millennia in Europe the idea of conquering the lands of darker skinned people has been a thing.