Quote from jackedup:
Economics. The vast bulk of China's economy are exports. What do you think would be the benefit to them if they allow the yuan to rise? It would severely hurt their economy. They have no reason to revaluate except to help the US. And they sure as hell aren't going to do that.
Economics, you have to print shit loads of paper money and throw it in the market to counter that natural pressure to make your currency rise. Sooner or later you drown under the flood you created.