Why do book critics despise adverbs so much?

They can get a bit excessive/annoying if used too much – they can become a crutch so that a writer ends up using a phrase like "walked slowly" instead of a more interesting single word like "meandered" or "strolled". Basically, it's in the interest of making language short and sweet, and using better vocabulary. However, I do hate when people keep harping on it all the time as if having adverb-less prose were the most important thing in writing.