I just calculated, based on the global hashrate, that bitcoin mining uses between 0.1% (if all miners are S9's) and 1% (if a mix of newer and older hashers are in use) of all the electric energy generated by mankind.  That much energy would run some respectably-sized factories (or cities).  Maybe China wants to keep power directed toward industry?

Also, Jørgen Randers mentioned in 2014 that Norway owns something like 10% of all the stocks in the world (he says, "filthy-rich Norwegians").  That's what they did with their share of the North Sea Oil earnings.  It sort of makes sense that a couple of Norwegians would acquire so much mining hardware...