[Pentegarn]

It was clunky, had zero battery power, a spinach green screen, and yet....

It had Tetris on the go, which seemed to win the day at that time. The only handheld competition that was relevant (sorry Lynx and TG16 portable) was the Game Gear, and as bad as Gameboy was on batteries, Game Gear was way worse. So Gameboy won by simple default. Also Nintendo had nearly godlike status in the vidjamagames market in those days so if you slapped the Nintendo logo on a bucket of vomit people would have shelled out hundreds of dollars to buy it

As I recall the initial library for Gameboy was poor, other than Tetris and Super Mario Land there was nothing of note going for it (though I may be forgetting a title or 2)