How could you say .CO failed? Must every name ending be as successful / widely used as the legendary .COM ? Or, are you assuming the .CO registry is not profitable as a business?
Also consider the fact .Co registration price is typically 2-3 times higher than .com domain names. Otherwise, registrations and usage would be way much more higher.
The reality—like you've confirmed, that .Co is the most common non.com extension (previously a ccTLD), is evident enough it is successful / did not fail. It wasn't acquired for $109 million by neustar for nothing.
A good example of a failed extension is .mobi as many on NP would agree, but not .CO!