I think the main factors are 2:
- optimization of development time
- audience reached (possibly oriented to buy games/apps)
So, I'd say the best routes to follow are:
1) DirectX runtime -> PC and Xbox
2) Macos runtime -> Mac/iPhone/iPad/iYoursister/ecc.
Secondarly, i say Android: still far more revenues on Apple Store, but the market share is growing bigger and bigger.