Spotify and Pandora work with more devices than itunes. Many people on facebook and other social sites already use spotify and pandora. Some cars use pandora. Also, itunes is more expensive than spotify or pandora, and this will make it less popular along with the fact that it doesn't work with as many devices.
Spotify and Pandora are dedicated clients that provide a maximum quality and greatly interfaced radio program. Itunes is multi faceted and though it has the Apple market, does not have the ability to overtake these other dedicated pieces of software. It would be like expecting a buffet to serve better lobster than a seafood restaurant in Maine.