The fact that each Chapter is composed out of 1.000 Marines. Which means we have 5.000 Marines. Whereas the Eldar, Chaos Marines and Especially Orks have Numbers FAR outnumbering those of the Marines because guess what? Marines doesn't have the Role to fight planetary Campaigns on their Own since 10.000 Years. They became highly specialized Strike Forces to support the Main Fighting Force of the Imperium and make the essential Breakthrough at Crucial Points (and take all the Credit for the Campaign in the end of course!) So... if EC keeps it's MMO premise.. it's a Fight between Chaos, Eldar and Orks after.. let's say a Week!