In a FFA, there is no incentive to attack.
Quick example. A takes the initiative to attack B. Both fight it out, and both are low on forces due to the fighting. C comes in with his army, and crushes both A and B.
So if you sit back and let everyone else kick each others asses, all you have to do is run in and pick up the pieces. Of course most people know this, so the entire game becomes one long drawn out stalemate.