It's one of the longest lasting consoles, and has one of the largest libraries of games, possibly the largest.

Backward compatibility with a huge library of games, along with long battery life, are what has kept the Game Boy popular when Nintendo releases new versions of it. Moreover, though the Game Boys Color and Advance could easily stand alone as systems, because of their obvious improvements on the original Game Boy, the Game Boy Pocket was a truly unnecessary release (especially since the Color came so soon after), and would have been nothing without the Game Boy's established back catalog of games. In any case, all these new Game Boys have been severely lacking in an important area...

Sure, a color display and more power are nice, but where's our backlight?