Auto makers in the US are required to keep the average mileage of their vehicles below a certain standard. This means that, to produce a lot of inefficient vehicles (trucks, SUVs, etc.) they need to produce a quantity of fuel efficient vehicles.
Producing vehicles that use 0 gallons per mile has the added benefit of allowing them to make more gas hogs. This probably isn't a huge part of the reason they made the volt, but it would be a contributing reason.
Staying relevant and in the press would be another factor.