Perhaps too many people turned on the tennis, saw
THIS on court, and thought they had The Cartoon Channel by mistake?
Seriously though, I think its simply because they had two players who were relatively unknown to the wider public. Its not just hardcore tennis fans who tune into a GS final, anything but. If Agassi or Sampras had been playing the final, I bet there'd have been higher ratings. If Rod-duck reaches the final again next year, I'm sure he'll attract more viewers too.
Or maybe people got all tennis-ed out in the few days before as the rain finally stopped?