Splintering happens a lot in fantasy. Katherine Kurtz's Deryni series, Bakker's Prince of Nothing, etc. In this thread alone, we've come up with say twenty bestselling series that aren't really matching these complaints just to start. Irritation with some novels is natural, but we're talking about thousands of fantasy releases, sixty percent of which are not secondary world and of the forty that are, numerous different kinds of novels. So as tempting as it is to cast a wide net on the basis of a handful of books read and a possibly incorrect analysis of book covers, it makes more sense to say that the Middle Ages are a popular setting for secondary world fantasy, rather than that fantasy fiction or one sub-set of it is "stuck" there.