I suspect at this point, everyone who currently owns an iOS device and wants Angry Birds has it. Their sales are mostly to new iOS device owners while new games can sell to everyone.
Angry Bird as a brand grew also outside the App Store. As long as you don't factor in the money made with licenses on cups, toys...; this analysis based only on App Store revenue just falls short.
Simply put, you don't need to make money in the AppStore (and hand over the a huge amount of that to Apple) when you can monetize outside of it.
Yes, the reason why Angry Bird is not in the top grossing app is because they already dominate the market. This metric is based on the number of new install on the last week or so. The fact that they are still #2 of paid app (also a metric on the last month or so) means that they monetize pretty well with in app purchase already.
Perhaps Angry Birds is dropping on the pay charts because people are finally bored of the franchise? How many years and different ways will people pay to shoot slightly different pigs with birds?