Cheeky bastard, I spat out my coffee at "Deadbox". XD
As I understand it, it's largely down to lifestyle; the Japanese work very long hours, so a portable system is more convenient for them because they can play it on their commute as well as just during the very brief amount of time they have at home and not sleeping.
I can testify to that... Been there recently and since transportation is highly developed, Japanese always favour public transportation rather than using cars. This gives them an opportunity to keep working, or use mobile devices.