Why didn't China's economy surpass that of the United States by now?

A couple of decades ago, people were predicting that China's economy would surpass that of the United States as a whole. This didn't happen for whatever reason, but seeing as the Chinese population is 1.4 billion people, and the US has a population of 334 million, they have more than 4x the human capital we do. Why hasn't China's economy surpassed that of the US yet (and if it ever will)?