I, too, love J-pop more than western pop music, although I do listen to some of my native singers and actually enjoy them. But those aren't exactly the well-known artists. I'm not exactly into J-rock, but that's mostly because I'm not into 'louder' music in general.
It's also a bit of the exotic twist that attracts me to Japanese music. That, and I often find that they can actually sing and dare to experiment, as opposed to some western artists. (Not all, mind you.)
At the moment, I'm really into KOKIA, a J-pop singer with a lot of different sides. She's also sung in English, French, Italian and Celtic. That should rule out the fact I only like J-pop because it's sung in Japanese. Although it is what attracted me first to J-pop, it's not #1 requirement to me anymore.