Why do you think the series failed?
From AV Club:
"This is the season finale of Colony, but as of yesterday, it’s now also the series finale. The critically resurgent but chronically under-seen show is wrapping up its third season with a cancelation notice from USA. The current arc has felt like two years’ worth of story in one, as the fight against alien overlords has turned its attention to the messy realities of the various human factions trying to carry on some semblance of life within the Seattle colony—and the secrets contained therein. Tune in to find out just how much of a cliffhanger we’re going to be stuck with as the grace note of this flawed but fascinating series."
I blame the lack of ALIENS. Key to a story about alien overlords, yes? You can't just dribble out little alien teasers every few episodes and keep your numbers up. I honestly think the writers didn't know what to do next so wasted our time with family interplay and looks of worry. So many good teasers of things to come without ever getting there.