Gabe Newell dropped some fried chicken on him and killed him. How? Gabe orders the 100-pound bucket, that's how. Outraged, Gearbox detached from Valve and demanded compensation for the tragic and greasy loss of Corporal Shephard. After a drawn-out legal battle, Gearbox raked in the spoils of victory, which included, but were not limited to: Gabe Newell's personal wardrobe of single-color XXXXXL t-shirts, and all intellectual rights to the aforementioned character Adrian Shephard.
Either that, or Gearbox just moved on after licensing Unreal Engine 3. I don't remember exactly.