Because the Iowa caucus is the first Presidential primary and it gets old gas cars off the road quicker by gunking up their fuel systems, allowing manufacturers to sell newer vehicles.
Out in the country is the only place I can still find 100% gasoline.
But mandating a certain percentage of ethanol ensures a minimum level of demand, preventing that problem.
I think there is some kind of presupposition here about how voting correlates to policy as enacted. What I would put forward instead is that regardless of how rural people vote, it's Big Ag lobbying and setting the agenda/policy.