The reason is that those who wish to have something to kill time with without having to think, and who don't require well-written plots without holes and shallow characters, do still enjoy her books and that is fine, but what others do not like is that her books are hyped to a ridiculous degree and if that is the quality of books needed to top all charts everywhere, that makes authors less likely to bother with the higher-quality literature that many in the book community is striving to find to be able to enjoy reading.

Similarly, how the fact that movies are nowadays dumbed down, with idiotic characters and comedy that is apparently enough to amuse the big audiences, means that quality movies with well-thought out plots are getting rare, and how AAA video games are buggy and poorly developed because people buy them anyways whether or not the quality is great.

So normally, other people having different preferences doesn't hurt anyone, but in this case people enjoying and hyping dumbed-down stuff in droves is actually hurting the market of "quality" entertainment, and that makes many feel rather angry.