There are plenty of great Japanese fighters. But most of them fight closer to home. The Asakura brothers, Kyoji Horiguchi, Yuya Wakamatsu, Tatsumitsu Wada, Hiromasa Ougikubo, Shinya Aoki, etc.
I think there's a number of reason why you don't really see them in the UFC.
1. Culture barrier. Japan is one of the more homogeneous society in the world. not very interested in moving to Florida to train. I'm sure UFC are offering peanuts.
2. Japanese fighters populate a less popular UFC demographich. If you want to build a BADASS Flyweight/Bantamweight division with no-names, then Asia is the go-to region. However UFC does not. Horriguchi was 6-1 in the UFC and they just let him leave.
3. Japan (and perhaps more so Korea) has this Bite-down style: Gomi/Zombie/superboy/maestro, all value grit and chin way too high, and that does not work the moment they fight a tank of a fighter who does not get hit, and has the power to break any chin.