A guess of mine grabbed out of pure air is flash has >99% of the market. I've not seen one single site ever of those I regularly visit that uses silverlight, and none of my infrequent visitors/random stumbles either, excluding those sites I checked out that were linked from silverlight.com.
Personally I'd love for flash to die a fiery horrible death though, because it's the scourge of the earth as far as I'm concerned. No browser plugin in the history of mankind has caused so many crashes as flash. Nevermind it's slow, from a stability POV it's crap, plain and simple. I still get several crashes or other glitches due to flash every week, thankfully it doesn't take out the entire browser anymore in IE8 like it did in previous versions, but it's annoying enough when it happens as IE stops responding until the tab has been recovered...
I am not a fan of flash either but I think HTML5 is the first step in a long road to being flash free.