It was due to a direct order from a pope (George/Gregory #??) to not destroy “high places” but to use them to gain more traction from the locals. There’s actually a still intact letter written by him, sent to the guy in charge of converting the pagans in scandinavia. There is no known better representation of this than Norways(?) oldest stave church. It is a christian church, built/ordered to be built by christians yet the inside is full of pagan themes. There is even a tiny one-eyed Odin a top one of the pillars. *The stave church, IIRC, is Borgund. Really famous one. If you go, bring a flashlight and maybe binoculars and look at all the tiny carvings not usually seen by people up high in the ceiling and pillars.
It was a strategy of conversion from “respect”. As if to say both religions are the same; Odin is just a ‘mask’ for god. Our religion is closer to the truth. It worked very well.
Sauce: I did a whole course on this because a bunch of historians asked the same question.
*Found the letter! It was by Pope Gregory I, directing abbot Mellitus c. 597 CE:
Tell Augustine that he should be no means destroy the temples of the gods but rather the idols within those temples. Let him, after he has purified them with holy water, place altars and relics of the saints in them. For, if those temples are well built, they should be converted from the worship of demons to the service of the true God. Thus, seeing that their places of worship are not destroyed, the people will banish error from their hearts and come to places familiar and dear to them in acknowledgement and worship of the true God.