Same reason anyone buy gold: they think the economy may get in trouble in the future, and they are preparing for some fiat strong devaluation. Difference is that they have the secret service and an army of people to make economic plans and collect information, and ways to e active agents.
All the same China could opt for a compromise position, something like Chinese Bitcoin; a digital exchange currency backed by gold. I'm actually surprised someone hasn't done this already and China would definitely have the resources to make that happen.
Hashdollar was a try in that direction, and was a complete failure. But also has not been launched by a major economic power.
Ecuador launched their own crypto, while turned holding , buying and selling any descentralized currency illegal
Depending of the results of the new Ecuatorian law, it may influence other countries to do similar.
http://www.coindesk.com/ecuador-bans-bitcoin-legislative-vote/There is nothing that indicates that the new digital currency of Ecuador will have any of the characteristics of a cryptocurrency. It is highly unlikely that it will be decentralized, transparent, algorithmically-constrained, or that users will be able to control their private keys.