developer here, chiming in.
I think it has more to do with the gaming APIs (openGL) being the same between iOS, Android and even webOS! For WP7, AFAIK, the API is largely built on DirectX and XNA, which are fundamentally different.
So from Rovio's perspective, they can release a game across openGL platforms with little-to-no code changes. For WP7, there would be far more development. There's a cost associated with that, so maybe they don't think it's worthwhile.
TL;DR the cost to port between iOS and Android is close to $0. The cost to port to WP7 would be tens of thousands of dollars.
edit: not saying it's OK, just giving perspective.