4.4.3. Doom : Becoming Open-Source at the Right Time
When the game "Doom" was released, it had ground-breaking technology and superior playability.
Keeping the source code of the engine for itself was an economically sound choice for i.d. software.
As similar engines came up (and i.d. improved upon their engine) the value of the Doom source code became very little.
It was then open-sourced under the GPL, as were subsequent game engines (such as Quake) and now i.d. is better off making money from selling game worlds for players to play in with this engine.