I think part of it is, the really good gear is more memorable. If you drop $500+ on something, and end up liking it, you are more likely to review it. If you spent $20 on a crappy pair of booties, you just get something better and move on. Writing a review takes time and effort, so why should you waste it on something that wasted your time and money?
With that said, I thought back to gear I didn't like, and posted a bad review for something. It was nothing big, and most people here probably already know to avoid it.