Why? They are not readily available. There's any number of guys on this site interested in biodiesel, propane, ethanol, E85, but most people can't fill up at the corner station.

In a perfect world your local "gas" station would have gas, diesel, propane, biodiesel, E85 and ethanol pumps. Maybe even others. You could run your truck on whatever you wanted, or with 2-3 trucks run whichever fuel is cheapest per mile this week. Say you have a F150 with a 351, a diesel F250/F350, and a flex fuel Ranger. You could handle just about any situation.

Opec's cutting production back and crude prices go through the roof. You jump in your 12:1 compression 351 and run straight ethanol.
There's a drought, ethanol prices go through the roof. You run gas in your ranger or diesel in your F250.
There's a drought and Opec's acting up, so run biodiesel in the F250.
There is almost no situation where you would be forced to pay $3 per gallon. Less people would be using gas and as demand goes down so do prices. More renewable resources would be used helping the environment. In fact, if you are really environmentally active, you could use whatever you thought the cleanest fuel was regarless of the price.

Most of us have more than one car. Most of us like building and modifying our vehicles. This really is not a impossible situation. The only reason we can't do this is the lack of availability of alternative fuels at "gas" stations. As we've seen with E10 when congress acts they try to throw a blanket solution at everyone and screw up supply for months causing high prices. How do we get them to realize that consumers and the market will fix all these problems if they'd stop throwing blankets over us and give us half a chance?