The US had a control center near Chicago go down when a disgruntled employee or contractor set fire to it or a computer room associated with it. No backup there either.
I imagine it's a cost thing. Imagine how much it would cost to have a fully functional room capable of handling the needs of an ATC zone the likes of London's Heathrow approach or Chicago's O'Hare? Not only is there the initial cost to build the room but the cost every time there are system upgrades/updates, 2x of EVERYTHING.
Now, what they SHOULD have are double or triple redundant power feeds, coming from different segments, and backup generators in case ALL incoming feeds go down. That takes care of the power issue but doesn't account for loss of the space such as happened in Chicago.