Why should I do that?
1) D&D is based on Medieval folklore and fairy tales.
2) D&D has clerics and druids that get their spells from deities.
3) The technology of D&D is Medieval, not Ancient as in Ancient Rome and earlier which was the time of those pagan deities.
4) In 1100 AD, In Europe, the Ancient Gods are in decline or a thing of the past, the new religion is Christianity and Islam, but I am not bringing up Jesus. Both religions worship one God, what they believe about that God is different, but I am not getting into that. What is true is that the pagan gods of Europe belong more in the classical period and earlier, the days of the Roman Empire and earlier.
5) If I'm going to base this campaign world on history and have recognizable place names and cultures, then I have to use Christianity, Judaism, and Islam. I don't have to get into the specifics, all I need to do is explain how the Christian, Jewish, and Muslim clerics get their spells from. They believe in different things about God and everything, but the one factor they have in common is a belief in a single God, so I'm going with that. This God does not typically make personal appearances, Jesus is not around in this time period and neither is Mohammad or Moses. So I don't have to involve them. All I really require here is a mechanism for monotheistic clerics to get their spells, their agendas are their own. So I have a force for good and a force for evil, that evil force is Satan, all three religions have a Satan or something like him. I don't care about the minutia of each religion or what drives them to go to war with each other, such as in the Crusades, it is a conflict between humans, not between gods or a God with himself.