Because it is a listed, traded commodity where prices are external of traditional supply and demand paradigms of extraction and manufacturing. There is a lot of capital yield in commodities that have nothing to do with the underlying asset's use in these markets.You'll find that even when metal commodities are in surplus phase, they still hold their value and even see positive flows of investment, resulting in an upswing in prices. It is no longer a matter of traditional pricing of fundamentals, but rather about where an investor can find yield.
In a market like China's their investors' primary objective is to identify capital appreciation as that's traditionally been tough to get with the banks, so they have cash to move - so where do they move it? Commodities. This allows commodity prices to be manipulated, external of the fundamental market drivers of the underlying asset. Now imagine that scenario played out at 20X to 100X margin, and they can do more than just manipulate prices - they can dictate them to an extent, which is partly why platinum and gold have been resistant to traditional fundamentals challenges like supply and demand.
EDIT: another way to look at it is that supply and demand equilibrium prices are no longer important when so many marginal investors are putting their cash into commodities. The manufacturing guys have no choice but to buy at market prices, and this is how the market sustains its pricing to a large degree, as there is surplus that is sold at these prices. And the surplus phase continues, while marginal investors drive the prices. They wield a bigger stick in the commodities market. The only way to prevent this imbalance from taking place is to remove the option to purchase metals on margin, but that is not feasible as metals are often correlated with interest rates, which makes them a cheap risk-mitigator when traded through a treasury function on margin. Remove the option for margin-traded metals and you add massive risk to the market and cause a liquidity crisis, as well as slowing economic growth, adding pressure to spending, contribute towards massive spikes in inflation, and much, much more...