The MLS, along with real estate sites such as Zillow, Realtor.com, Trulia and the like, only target those people who are actively looking to buy a house now. These potential buyers have raised their hand and are now identified by these sites as people who are actively looking to buy a house, but as the sales data shows, there are simply not enough of these active buyers compared to all the houses for sale, particularly in the price ranges above $500,000.
Yet, the marketing approaches 99% of all real estate agents use to find a buyer only reach out to those active buyers already looking to buy. And with more houses on the market than "active buyers", many sellers will not be happy with their end result.