I'd say that Price is the biggest contributing factor to the low sales.
From the article:
But the biggest factor may be the price of the Volt. At $33,500, the Volt is not cheap.
But their numbers are wrong.
A look at GM's website shows that a base model Chevy Volt will run you $39,995
if you want it fully loaded it will run you $46,395
http://www.chevrolet.com/tools/byo/byoCustomizeVehicle.do?year=2012&brand=volt&evar10=byo_path_buildbytrim&pvc=500