Firstly, steel futures trade on the London Metal Exchange, maybe on other exchanges as well. Perhaps your 32 number was limited to one exchange? I've not researched all the exchanges, but I'd wager there are far more than 32 commodities for which futures contracts can be entered into.
How is it determined which commodities get futures and which don't?
In general, demand. If there is enough demand for futures on a certain commodity some exchange will bother to support them. In some cases, demand may exist but government regulation could prohibit a futures market for a commodity.
Why didn't futures contracts exist on bitcoin until recently? Lack of demand, once enough people wanted them an exchange obliged.
Why is there demand for futures contracts on some commodities and not others? Well, cattle and hogs are far more valuable in the US than goats, and just like with crops, supply of both can be disrupted in any given year by weather/disease. You could simply say that price variability and high overall value in the economy are what create demand for futures contracts on a given commodity.