probably because it has 88 keys...
Actually, 88 is not the reason. The reasons are sound, versatility, weight, and price. I've been in the market for something different than my s950. I've tried Korg and Roland and couldn't find anything I really liked OR could afford.
I don't know if the Privia will be as versatile as the Yamaha, but since I rely on a lot of midi files, I'm hoping I can continue to do that, play some styles and enjoy playing a nice piano.