Doom pioneered the FPS genre in the USA. So it has a cultural significance. I used to go to my friends house to play it. You deal with a whole range of aspects such as grapchics, sound, networking,.. Etc. So it's a fun challange for hackers to try to get it to work in the most strange of ways. In addition it's a simple goal (with a complex path). So it helps with the focus, and most importantly the fun programming challenges involved in making the game work.