DoW3 was management of relic completely losing touch with playerbases of DoW (DoW1 players are not the same as DoW2 players) and their attempt to cater both spectacularly failed on both fronts, the game had absolutely nothing to do with CoH3 or vice versa.
As laid out by Katitof, what little evidence we have suggests that Relic have been in a long slow death spiral for quite some time. They ruined the DoW series but managed to turn out a fairly decent, if unremarkable 'new' Age of Empires game that has probably helped keep them afloat.
Tiger is right of course in saying that none of us know the precise reason, we're all just speculating. But for my money it's probably down to a combination of management incompetence (I think they fired and replaced the main guy shortly after COH 3's launch), a much lower budget to develop the game with and the fact that a significant number of the people involved with the original have now moved on to better things.