Nintendo cares about battery life above most other concerns. The Game Boy was not the most capable handheld when it was released. It was severely underpowered compared to the hardware that was out there. Atari released the Lynx a few months later. Sega release the Game Gear the following year, which was essentially a portable Master System.

However, the Game Boy ran forever on 4 AA batteries. Which is part of the reason why the original Game Boy has outsold every other non-Nintendo handheld gaming system except the original PSP, combined.