You know how people in this thread have said that Japan likes tacking things on to their existing belief set and making them all compliment each other instead of fight against one another?
Christianity has an incredibly storied history of destroying other religions and assimilating the parts they like and attributing it to the One True God. So storied that it's a major part of the bible, because when you look at even the old testament it's about destroying a thriving polytheism. It is fundamentally incompatible because Christianity is a destructive theology in practice.
You literally had an example of how this isn't the case in your own post. The fact that Christianity/Platonism, and really monotheism more broadly, has tended to absorb other bodies of religious thought and practice doesn't mean it's not compatible with other bodies of thought in the abstract in fact it suggests that it very fundamentally is.
As an additional irony the example of Pure Land Buddhism in Japan that you're drawing on is just another example of the exact same thing happening with different names. Hell the way that even older Japanese religious practice has fused with Ainu practices is as well.