1. Nukka, Cadia is blow'd the fuck up. The gates holding back chaos are gone, the eye is expanding. Armless the harmless just curb stomped creed, who had backup fro trollzyn and some other losers.
2. Chaos thrives off bloodshed (khorne specifically), hedonisim (slaanesh specifically), Mortality (nurgle specifically), and ambition (Tzeench specifically) the imperium gives this in spades and DE have been going out of their way to feed slaanesh as much as possible so it doesn't nom them instead.
3.Nids are the only threat to their food source, tau can be corrupted (farsight conclaves) and the eldar just wanna cry about how they "was kangs".