Because flyciderman there is no good alternative at the moment. What's needed is a website like Rightmove and others that allow individuals to market their own homes. There are a few like Propertyladder but, unfortunately, they dont have a professional look or feel about them and therefore dont attract the same number of properties that sites like Rightmove do.
Once someone sets up a good alternative, and are prepared to spend money advertising it to get it known, estate agents will become a dying breed.