There are a number of ways trading stocks is easier than commodities:
But the main and most important reason is that over long periods stocks in general will tend to outperform inflation as you are investing money in enterprises that generally try to become more productive over time. Whereas commodities in the long term tend to rise only at the pace of inflation (this is kind of the definition of inflation actually). So even uninformed investors that pick stocks at random will generally do better than someone doing the same in commodities even before the higher commodities trading fees are taken into account.
Also your orange example may be harder than you think. Once the news that a drought is an issue the price of oranges will almost immediately change well before the oranges come to market! So unless you can predict the drought before anyone else can you won't be able to make money this way.
One reason why you may have gotten this advice is that stocks have an expected real return over time, while commodities do not. Therefore, when gambling on individual stocks, odds are in your favor that they will ultimately go up over time. You may do better or worse than the market as a whole, but they will likely go up as the whole market, on average, rises over time. Commodities, on the other hand, have no expected real return. It is more zero-sum. In fact, after costs, a real loss should be expected on average, making gambling in here more risky.
Its the relative leverage available to retail traders between the two. In the US one can trade equities with 2:1 leverage while with commodities the leverage can go much higher.
Combine this with the highly volatile nature of commodities, and it makes losing BIG too easy for the average trader.