Why does everyone assume that Iowa voters are all farmers? Farmers make up less than 2% of the population of the United States.
No one assumes that. But in Iowa, the farmers wield influence far out of proportion to their numbers, primarily because so much of the economy in the state is dependent upon agriculture. In other words, it is not just the farmers - it is also the farming equipment dealers, the people who repair that equipment and sell other supplies to farmers, the people who work on the farms; then the people who sell groceries, homes, cars and other goods to the people who rely on the farms for their income, etc.
That does not justify a candidate pandering, but does help explain why the farmers have such influence.
Ethanol is an entire industry and numerous subsidues are given to farmers. These subsidies employ many people in various companies that make and transport the ethanol. Not to mention the local corrupt GOPe politicians.