Anything with starch/sugar can be fermented into ethanol.
Just when you are paying by the truckload/trainload/boatload you want a starch source with as little water as possible.

#1 you don't want to pay to ship 100 tons of "water"

#2 you have to remove the water from the ethanol in the end

Sugar beets and potatos work well for ethanol.
But when you buy 100 tons, you get ~50 tons of water.
Those options are "juicy".
( same with grapes... in wine, you of course can leave the water in... )

When you buy 100 tons of corn, you get less than 15 tons water.
( field corn is less than 15% moisture )