In a traditional supermarket, you have one company that owns and operates the entire store and in essence, the entire selection of products available to you. In a wet market however, you have one giant building that is separated into hundreds of stalls. Each stall is leased out to individual operators to sell their own produce, meat, or what have you. This way you get a wider range of products at a very good price.
source: http://www.goodfoodandtravel.com/hong-kong-wet-market