Because players who never plays Doubles but want to try it with a friend of them who's rnaking is not high enough to give them a WC or a direct place will never get in and they won't get the WC. So they will not play.
stosur is just one..for black, huber, VRP, Suarez, Raymond, Peschke, Husarova Stubbs, Stewart and many other 80% of the prize money (and even more in some cases) coming from the doubles
But the important thing is that young players won't be allowed to play doubles unless they are given a wc. And when you want to play with someone who is not a top doubles player you can't. I don't give a shit about prizemoney.