Why is Angry Birds so popular?

[UserNameGoesHere]

I don't understand it. I really don't. It's just Crush the Castle or Fragger but with birds and pigs.

And those games were based off of earlier games such as Worms, which itself was based off of the earlier Scorched Earth, and if you go further back you have GORILLAS.BAS and there may even be stuff before that.

Basically, there is nothing original nor even noteworthy about Angry Birds and it isn't even as good as some of the earlier games it knocks off. So why the popularity?