The climate in the majority of the United States isn't good for growing sugar cane. For Brazil, using sugar cane makes far more sense. For the US, it may make sense in some areas, but not most of the country.
Using corn doesn't make a huge amount of sense either because of the poor yield, but ethanol is being forced on us before the technology to produce it efficiently is ready. Those why are trying to gain wealth or power though pushing the global warming scare on us, aren't willing to wait until it is economically viable.
My farmer brother-in-law told me it wouldn't work 10 years ago when they first started looking into it!
It wasn't viable at all ten years ago. It is only marginally viable now, when heavily subsidized.
If making ethanol from cellulose become commercially viable, then it may actually be a viable fuel source, but in that case, corn isn't likely to be the best crop.
Making ethanol from corn might have been reasonable in order to test out and develop the technology to use and distribute it. It might have even found a market with people who are willing to pay a large price premium because they place additional value on using ethanol as fuel. Consider the number of early adopters of hybrids before they were economically viable.
However, right now the corn ethanol production capacity is growing far faster than demand, and the producers faith that the government would bail them out with subsidies may have been misplaced because of the rising costs of food.
However, if ethanol from cellulose becomes viable the current ethanol producers might be positioned to take advantage of that boom, if they survive that long.