Having not played it, is there something specific about DOOM which makes it so easily accessible to these sorts of hacks?
2. It was also open sourced a very long time ago by id software, which enabled a lot of tweaking by the community, and, therefore, a lot of knowledge.
3. Doom is a very famous game, having made a lot of success during it days. Everyone with even the smallest interest in games or computers that was alive back then probably played it.
4. Both Wolfenstein and Quake are also open source but the former is too simple when compared to Doom and Quake is too complex. As others have stated, Doom has 2.5D maps whereas Quake have full fledged, 3D maps.
All of this together surely makes it a good candidate for this type of experiment
It is known to be high quality and hackable, by orignal design and later hard work of fans.