The problem is alcohol from corn is easy and the left over distillers grain is a valuable animal food. It is compact and easy to store. Common yeast does most of the work
Ethanol from grass at this time requires much different different process. Fermentation of grass does not yield significant alcohol. Manufactured enzymes are used, this is more expensive than corn alcohol. The left overs are not nearly as useful, not much feed value. More volume and weight to transport and long term storage is expensive too.
No free ride.