Game Boy didn't have state of the art hardware when it released, it had reliable hardware. Its competitors had color screen and what not; they all failed.
Common mistake the phone market is still making today.
People don't buy for the tech, they buy for the [games | experience | brand]
Yeah the original BW gameboy used a modified Z80 which was old even in 1989. It took 4 AA batteries and had twice the battery life of the 6-battery (?) Game Gear.