There has been an interesting discussion going on at various historical writers' blogs lately, generated by a discussion on an e-mail list about the differences between "historical fantasy", "historical fiction", etc. I should note that, try as I might, I couldn't track the URLs down; I didn't save them. They're in some of those blogs, though.
But the gist of these discussions was this: Why some writers(and readers) don't like fantasy, and why does(or deosn't) certain kinds of fantasy work very well. In particular, "historical fantasy", which is defined as historical events "assisted" by non-means, or "alternate history"(which should be self-explanatory) came in for criticism.
Just for the record, I do read fantasy. I always have. I can't even remember a time when I wasn't reading science fiction, though there must have been such a time when I was very young. So, in a way, I can't understand why some people don't like fantasy --- except that some people don't, and I happen to live with one. In this person's case, I can understand why: it had a lot to do with the opportunities she had to read much of anything in the rural community she was brought up in. Besides which, her parents never had much money anyway, and the public libr4ary system in that area could only be described as "crappy".
But the author who said she didn't like fantasy(and who writes historical fiction), was another matter. She just never did like fantasy of any kind. Not even what are mistakenly called "fairy tales", even as a child. I can sort of understand; there are authors that some of these writers absolutely adore, but I can't stand(I won't go into that here, at this time, however). A lot of this is a matter of taste.
Unless the person in question never really, psychologically, was a child, or else was brought up to think that you "put away childish things" once you get past a certain age. There are a lot of people like that, and it's hard for them to give up their long-cherished beliefs about what is right and proper. And maybe we shouldn't even try.
Be that as it may, the discussion was quite interesting, because it touched on the kind of material I'm writing. It's set in medieval England, and closely(at least as closely as I can) follows real historical events. But three of the most important characters are Neandertals from a refuge planet, where they have learned space travel from "sasquatch-like" beings. They also have certain abilities not found in "modern" humans, though I(sort of) posit that these abilities stem from genetic material found in the so-called "junk genes"(which aren't all that junky, apparently). So my Great Medieval Science Fiction Masterpieces With Neandertals(it's turning into a trilogy)may seem "fantastic" to some people, and therefore may turn them off. This, despite the fact that there is no other "fantasy" involved, no "alternate realities" or "alternate history", just the factual history. So what would you call my book? I like to call it "romantic science fiction"; there's no such actual category, but it's very "romantic" and is somewhat like romance in that most of the characters end up happily.
Finally, I can see why some people don't like "historical fantasy"; certain authors try to "play" with historical characters and situations by adding "magical" elements. And unless the author knows exactly what they're doing, this just doesn't work. One quite awful recent example was Judith Tarr's Rite of Conquest. The premise was utterly ridiculous, if you know anything about the period involved. And Tarr is generally a good novelist. Even I, who am generally tolerant of fantasy elements, since I like good fantasy anyway, had trouble with this one.
None of this really answers the question of why some people don't like fantasy, but it raises a lot of interesting issues about what readers will tolerate, and what they won't.