The Great Game is the constant struggle for dominance between the Chaos Gods. The Warp is not only home to the Gods, but it is also their battlefield. No God can ever win the Great Game, for if all other Warp Powers were obliterated, the Warp would become a still, unmoving mass and Chaos would no longer exist; so it is an eternal conflict. The full Great Game commands the almost complete attention of the Gods; a fraction of their attention will very rarely be fixed on the mortal realm, but only for an instant, as the Great Game is far more important.
The Chaos Gods will also rarely unite (roughly every few thousand years), if there is the need to do so - i.e., to thwart a potent threat or to take advantage of a rare occurrence. The rise of the Emperor of Mankind was such a time, and so the Gods engineered the wars of the Horus Heresy to bring about His downfall, distracting them for an instant from the Great Game. The Great Conflict that is played out amongst the Gods is also mirrored by their followers and worshipers in the material realm. This is what the Daemon Primarchs and other Great Chaos Champions are occupied with, fighting for the supremacy of their God, with untold power at their fingertips.