Angry Birds is a just a simple game that appeals to all. Good games always sell, especially cheap ones.
Is it revolutionary? No, but it does take some thought, it's not totally mindless.
Not everyone buys utility apps that may have longer developer times or where harder to make.
There are also games, and this goes for any platform, that are all bells and whistles with huge development costs that simply aren't that fun.
It seems every year, or even every few months there's a "it" game, then it dies down. Angry Birds will die down and the next big thing take the spotlight and the 8 figures.