system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5:How did id Software's Quake engine lose ground in the '90s to where the Unreal engine became the game engine of choice?
gpt
Unreal's Engine was, quite simply, available.

Id was massively successful at licensing the Quake engine.  The Q3 engine powered both the Medal of Honor: Allied Assault and Call of Duty, for example. 

The problem was the next major release to come from the studio was Doom 3.  As Doom 3 suffered long delays, so did the release of the next id Engine.  

Studios were faced with the choice of buying UE2, which was available to them immediately, or waiting who knows how long for the Doom engine to be finished and released.  As a result a lot of studios went with Unreal. 

When it finally did release it had rigid hardware requirements that prevented older graphics cards from working with it, that competing engines didn't have.  It was also optimized for dark, spooky interiors (*a la* Doom 3) and had difficulty handling large, expansive outdoor environments, which meant it had narrow appeal.  This was the last version of the engine to be offered to external proprietary licensees.