To begin with, I'm an avid reader; and, with a Masters in comparative literature, I feel I know something about writing. Yet I can't count the number of times I've read reviews praising pedestrian books, while writers who display real originality and power are disparaged.
I think I might be able to offer a few explanations: 1) Ego. In boxing, a fighter will invariably praise an opponent he's beaten, while denigrating the skills of someone he's lost to. No one likes to admit someone is better than they are. A run-of-the-mill writer doesn't threaten a reviewer's ego . . . An exceptional talent reminds the reviewer of how glib and limited most of us are, including himself.
2) The bully syndrome. Put a punching bag in front of the average person and he'll probably hit it. Put someone who can hit back and he'll hesitate. The reviewer knows he can pummel a writer without much fear of retaliation. The writer knows that if he tries to counter in print, his letter either won't be published or his next book will be savaged to an even greater extent. So an author is pretty much at the mercy of the critics. This is especially true of unknown genre writers. Kick them and no one will notice that they might actually be good.