Colt and S&W seem to have dominated PPC and bullseye early on, so I'm guessing there's a bit of tradition involved.
The Ruger action also has a reputation for being a bit rough from the factory, and a rough action makes it harder to shoot accurately. That said, they clean up nicely, and I've been mighty impressed with the several tuned GP100s I've shot.
Grip selection for S&Ws is pretty extensive, so it's easy to find something that fits like a glove, which is important for wringing the most target accuracy out of your gun. My impression is that the selection of aftermarket grips for Rugers is less extensive.
Many PPC & bullseye guns and other target revolvers (e.g. Bianchi Cup) are tuned by gunsmiths who specialize in these disciplines and familiar with making S&Ws run well. Again, I'm sure GP100s can be made as good, but finding a gunsmith who can do it would be tough.
Some replacement Ruger parts aren't sold, so it'll have to go back to Ruger for repair if a piece of unobtainium breaks. But...and here's the rub...Ruger will return it to its original specs before sending it back. If your highly tuned PPC GP100 breaks a piece of unobtainium, then, you've got a serious headache.