Somehow, I think I missed a bit. The old civilization that was uncovered by UAC on Mars, apparently escaped to Earth after accidentally (just like the UAC) opening the Hellgate with their teleportation technology.
However, The Doom Guy/The Marine only managed to defeat the Cyberdemon because he previously found the Soul Cube, an artifact that was left behind by the ancient civilization on Mars.
I'm not sure if I may be mixing up the Doom film and the Doom 3 plot, but I think both statements hold for the Doom 3 story.
My point is, why would the old Martians have to flee to Earth if they had the Soul Cube, which was "wielded by their mightiest warrior"?