"The organisers have never really explained why dullness, deference, lazy thinking and received opinion might not thrive just as well in a negative book review as a positive one; nor why a glowing notice by a good writer should not elude these pitfalls. To do so would be to expose the fact that “hatchet jobs” are in fact far easier to write than admiring reviews; or that what they are really after is a piece of humorous, well-turned spite (and nothing too wrong with that, either)."
Writing about good things is harder than writing about bad things. Especially if you want to be funny.