The iOS/iTunes ecosystem is incredibly accessible. You don't need a credit card (important for minors; a huge market). You can buy credit everywhere. iOS users seem willing to part with cash.
Google Checkout on the other hand is not available everywhere Android is. People seem less willing to use it (eg 50k paid from this post). You can't buy credit in the retail chain. Google gives developers 95%.
The last one is actually a huge problem. Google's 5% will never pay for retail distribution (Apple's 30% clearly does).
Developers have come to the obvious conclusion: they'd rather have 70% of (potentially) a lot rather than 95% of much less.
iOS has the option of paid and ad-supported. Android only having ad-supported (realistically) is a major disadvantage.
The only way Abgry Birds could make it to Android is on the back of the success of iOS. I wonder how much money their experiment is really making.