But you can’t really be a global reserve currency when you maintain all the restrictions that China insists on in the interest of keeping control of its domestic economy. The dollar wouldn’t play its central role in global finance if the American government made it illegal to exchange it for other currencies in many circumstances or used legal prohibitions and aggressive interventions to keep its value from fluctuating in response to market forces.
In other words, China has wanted some of the diplomatic benefits it would gain if the renminbi became a more important currency abroad, without paying the price at home.