Why Is the PSP Such a Massive Success in Japan?

PSP World Writes:

"Sony may be struggling to wrest market share away from Nintendo in the handheld gaming space, but in at least one territory they have already achieved what some thought was impossible. The PSP has sold more than 10 million units in Japan alone, and the success of the console seems to be accelerating. During much of 2008, the PSP sold more on a weekly basis than even the mighty Nintendo DS, proving that Sony had well and truly sliced off a pie of the Japanese handheld gaming market."