Auction takes too long / is too heavy. The whole point of a convenience app is that it is supposed to be "just take care of this for me", not "let's spend time figuring out the bid/accept process".
You could see the listings sitting around in the marketplace for extended periods of time if you browse a bit. And I never used it because I didn't like that I had to guess how much the job was worth. It was clear that something in the model was broken