Some repeat of what others have mentioned but:
1. Status symbol
2. Japanese have easy access to US models or import models.
*If JDM offered same clubs ex. Tourstage X and Bridgestone J38 at the same time, Bridgestone would have lost significant amount money. So company decide to roll out equipment couple of years ahead of us market. Mizuno (except for Japanese specific lines) is the only one that does not do this.
3. Marketing is not global. You have to understand the market and in the US, you can't expect to sell a driver for $1,000 and make it into a hit product. But you can do it in Jap