Because the creator of the game said it was "ruining his simple life" ... guess not everyone is just out for riches and fame ;) .. he did say he could continue to create games but I guess if they get too popular he will pull them.
The creator said that it was too successful. I think that the games sudden popularity overwhelmed him. But that's just too bad because he would be making more and more money. I guess he doesn't care too much about that though.
It is no longer available on any app store unfortunately, but the legend will live on. In fact, there is a phone on ebay with flappy bird on it selling for over $3,000 right now. Flappy Bird is quite a valuable app now that it has been removed. If you have it already, you should hold onto it because to someone it may be worth something.