I suspect, my dear Ruth, that it is because you operate in a sort of grey area of the fandom. There's a great deal out there which is far more 'immoral' in Their terms, but it is so clearly Out There that it might as well have great big neon warning signs on it to Them. Whereas you, while you write "not your children's Narnia", write a vision trending much closer to canon which might very well BE the children's Narnia, until those other elements pop out and bap the reader right between the eyes.* Having been lulled by the more innocent aspects of the story, They are far more upset to find the worm in the apple, as it were, than they would be if they had picked a clearly bruised one up off the ground. There's also the fact that your writing is far superior in technical quality than, oh, roughly 90% of the Narnia archive, and there's always a certain resentment even among more open-minded fans at finding a fic that's well-written, coherent, structured, detailed... and about something you can't stand.

*To clarify, because I know you are sensitive to criticism, I don't mean that those elements are not well-integrated with your story, structurally or thematically. I just mean that to the sort of Reader under discussion, they stand out from the text.