Because money is power. Lets say the world uses bitcoin for everything, everyone pays their bills with it, their purchases... The more a government owns the more power they have, and they can also influence the price. Which can help their economy if they want to make things cheap or expensive. Like china has been already doing with yen/usd by de-valueing their own currency, they make exports cheaper, which helps them against US businesses and other competition. So the more Bitcoin china owned would actually do give them control over some things, as the bitcoin price.