Flappy birds got removed because Nintendo thought the style (pixel pipes) looked too much like their Mario Cart game. The creator of Flappy Birds also wanted the game to be relaxing, not strongly addicting. He got 50,000$ a week by just the adds and it only took him 3 days to make Flappy Birds.
Nguyen, the creator, was receiving death threats and suicide tweets. He said that "Flappy Bird" has become an addictive product, and he thought it was a problem so he deleted it, thinking that it would solve that problem.