Why? Because console gaming has been killing the game industry since the ps2.
No, this is not the reason. Here you can find an article by the name: "The PC is Rapidly Becoming the Largest Gaming Platform "
http://www.shacknews.com/onearticle.x/58497The reason are the online channels. The PC is just more flexible and entered some new markets. Boxed products are not the future.
But what happened to 3d Realms? we can only guess. Maybe too many managers, no milestones, bad project management?
