The DS built a new kind of gamer.. one that the industry never new existed and from that its software library became huge and now nearly all types of games can be found on it..

And many Japanese schools use the DS as an educational tool

Well, it's my understanding that the DS is very inexpensive, coupled with the mass market appeal.
It gives off a perceived value that seems to be irresistible at the price. That's just my input, though. :P

the games

Because the DS provides fun and entertainment on the go at a reasonable price. Also, the controls aren't (always) utilized by buttons but rather the touch screen thus providing a new interactive way to play video games. This appeals to many.

The Japanese don't seem to care about the hardware specs or multimedia capabilities of their gaming systems, just the games.

Because it has great games and brought a new concept of gameplay never seen before. That's why it succeeded.

Because it's a kickass system.