Philip Said:

Why is the serpent/dragon so prominent in ancient mythology and mainline religions like Christianity, etc?

We Answered:

Well you've done your research. Learned some good stuff here.

Most countries would therefore make out that there were some ancient serpents before man that where great beings maybe gods. This serpent worship probably developed is through ancient discovery of dinosaur bones. There was one myth that explained that giant snakes called the Kadimakara came from the heavens and were killed on earth leaving their bones behind.

European/ Middle Eastern counties took a different view that dragons/ serpents are something sinister not majestic or highly worshiped. The European/ Middle Eastern countries became very interested and involved in improving external conditions and so worshiping snakes was abandoned as primitive living (along with nature worship). Images of serpents and the dragon became a way of saying that the new modern way of living conquered the old such as in St. George of Christianity defeating the Dragon of primitive paganism.

This is just my hypothesis.