Why are only certain commodities included in commodity futures trading?

In order to be suitable for futures trading, the market for the particular commodity should be competitive in nature. Firstly, there should be large underlying demand for and supply of the commodity. Secondly, no individual or group of persons acting in concert should be in a position to influence the demand or supply, and consequently the price substantially. Thirdly, there should be fluctuations in the commodity price. Fourthly, the market for the commodity should be free from substantial government control. Finally, the commodity should have long shelf-life and be capable of standardisation and gradation.