Because anything other than the Imperium's destruction is nothing more than a zero-sum game for Chaos.
I suspect it ultimately comes down to this fundamental question: do you feel that Chaos wanted something more than it had (in terms of what it was getting from Humanity) at the onset of the Great Crusade?
I do. Ephemeral and elemental, the Ruinous Powers are nonetheless sentient. They just don't strike me as the types to settle down and accept a "business as usual" status quo. The state of the Warp itself is defined by excess: heightened states of emotion reflecting bad things done on a grand scale.
The Emperor's arrival at a most basic level heralded the end of a period that was defined by Chaos' undoing of Human society. Wide-spread practices were introduced that minimized the avenues Chaos had for access to Humanity. There's no comparing, for instance, the preferred state of things--e.g., Cadia in the 31st millennium--with a planet where no religious, occult, or otherwise supernaturally-aimed activity is allowed. Taken to a galactic level, this would be intolerable to Chaos. No wonder, then, that they opposed the Emperor.
This state of affairs has not ended with the Emperor's incapacitation. Galaxy-wide, Chaos is largely denied their due. Sure, it's fueled by the violence that comes hand in hand with the Imperium, but that's nothing compared to (even indirect) worship and the actions that go with it.
Just my opinion, though. I'm not trying to pass it off as fact, since I think that's a nigh-impossibility where much of the 40k mythos are concerned!