Why Aren't all the top Games onboard on WP8/W8 yet?

I can understand that lack of user base is preventing some of the app developers from developing their apps for Windows Phone, but the same is not true for the Game developers.

WP 8 supports Unity, Havok & even the Unreal engine since its inception. WP8 of late also has stuff like Merlamade


Developers can just port their OpenGL apps to WP8. What more are the Game developers waiting for?? What more is preventing these developers to develop for WP8??