ESPN will show matches that casual viewers will watch... and unfortunately, most of the potential viewership are not tennis fans and do not really follow the game. So they will watch players based on name recognition and celebrity status. The Williams sisters have that. To their credit, some of it is earned by being pretty darned good tennis players. And of course some of it is the back-story, media attention over the years, etc.
The sad reality is that if for example Paris Hilton was playing an exhibition match of some kind, it would have a good chance of drawing higher ratings than many real matches.