Why does it have to be Mars?
I just recently started playing Doom 3 BFG Edition, and it got me wondering there's no shortage of demon attacks.
Why not the Moon, or some random asteroid?
Doom - Mars (Phobos and Deimos)
Doom 2 -Earth
Doom 3 - Mars
Doom 2016 - Mars
Only one of the 4 main games has a setting other than Mars.
What is so special about Mars that makes it prime territory for the invading forces of Hell to come through?