Unlike stocks, commodities traded on MCX, which are traded internationally, are based on physical demand and supply. And to manipulate anything in any market operator must have fair control over the supply/demand in short run, and since there are so many players spread over the world that its rare but not impossible to achieve this feat. Even oil cartels like OPEC are unable to manipulate price. There is so much money and players involved that I think any kind of manipulation if there can not go unnoticed.
But instances of manipulation are there, like in natural gas, where US govt has recently taken steps to curb that, sort of same thing happened in Gold recently, which I think was not a manipulation but simple game of demand and supply. People who hedge positions in currency here are big arbitragers who deal in comex-mcx arbitraging but they are definitely not operators.
So running a scrip is comparatively easier, logical and profitable than daring to move international commodities.