It hasn't become concrete enough to make sharp, striking, correct predictions. It turns out the real world is extremely messy. Complexity theory can produce specific predictions ("disease spread will follow a power law with a critical exponent of...") but they won't be quantitatively correct. Or it can produce correct predictions ("airplanes increase disease spread because they increase the global network's connectivity") but they're generally obvious, and certainly don't require the full formalism of complexity theory to get.

I don't think it ever congealed into a coherent field, say, with an established canon of techniques and a heirarchy of results that built on itself.