Christianity largely meant to replace local "pagan" religion/faith, so building on top of those sites or sometimes even just rededicating temples made sense from that perspective. People liked their traditions and wanted to keep them though, so festivals were sometimes just reinterpreted as Christian holidays, local deities were replaced by specific saints etc. That way, people could be converted more easily as the new religion didn't feel as unfamiliar and foreign.

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.

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.

Erasure.

Roman assimilation. Rome was the Borg of their time.

Recruitment and replacement.