That's an excellent question, and I find the answers so far unconvincing. My guess is more psychological / sociological: GM has none of the sex appeal / marketing that Tesla or even BMW has. For that reason the Volt is easily overlooked or disrespected.
I'll also add that GM and other car companies already have some second-life battery efforts underway (http://thinkprogress.org/climate/2016/05/09/3775606/used-sec...), which is a very cool way of killing two birds with a single electric stone (I'm surprised this story and other like it haven't gotten more traction online). The second lives of batteries are incredibly underrated right now.
By the way, let me also recommend Steve Levine's book The Powerhouse: America, China, and the Great Battery War, http://www.amazon.com/Powerhouse-America-China-Great-Battery..., which concerns the initial development of the Volt, among many other things. People who disdain the Volt should in particular read it!
IMO Tesla really nailed it with the S, many parts of it being thought of completely as an electric car from the ground up. Look at what they did with OTAs, the sled design, etc.