Image shamelessly linked from GameFAQs. 2 out of 5 Game Boy launch titles were sports games. I'm not sure what that says about Nintendo, but it does make me wonder why every platform got so many.
So this is Tennis, the final of 5 launch titles for the Nintendo Game Boy. The 2nd of 2 sports titles in the launch line-up, Nintendo of America must really have been banking on the popularity of sports games, because the launch line-up included 2 games, much like the Japanese launch included Yakuman, a mahjong game. In the same way that every video game console ever released in Japan has likely seen a mahjong game (or thirty), every game system ever released in North America is generally peppered with sports titles throughout the console's life span. The Game Boy was no exception, and it received both Baseball and Tennis.