Quite simple - a "designer" who has just made his first design won't list it as a "Professional corporate template design. BIN $5", which is easy to do with no fee. When there's money to invest, the marketing starts to work - fee paid must be worth the potential income, so an amateurish design is NOT a product that will benefit from being listed. It's better to spend a few hours more and offer real quality product for the price that we'll easily accomodate the listing fee.
That's how the marketing strategies and plans work. Apparently SP decided to get rid of the bottom end, questionable quality sellers, which shows their long-term plans as being the place for professional web developers. Good move.
There are plenty of low price marketplaces and SP don't have to compete with them. Their audience is also different, so they can charge for something that's being offered for free elsewhere.