My opinion is that it is due
My opinion is that it is due to a lack of marketing, and to a lesser extent the somewhat polarizing view people take of EVs and PHEVs (not to mention even simple hybrids). In California and some other coastal states, there are enough that people don't see them as unusual anymore and are therefore fine with buying one if it suits their driving needs. But elsewhere, IMO there is not yet a critical mass, and hybrids are often viewed (incorrectly) as 'complex'. (Technically, in a good series-hybrid you're just trading an automatic transmission in the transaxel for a simpler electric motor). I took the plunge and bought a Clarity and love it. The Clarity is a PHEV series-hybrid with about 50 miles of EV range, but it has a "backup plan" for when it runs out of electrons - like the Volt did. So I can drive my daily commute without any gas 90% of my time and the other 10% can go across country using gas fill-ups if I want. Aside from the usual (and perhaps over-stressed) advantages of "green" and "saves money at the pump", I have come to realize series-hybrid car technology like in the Volt and Clarity is mature and the driving experience is so highly refined (good smooth torque, super quiet, etc.) that I honestly don't think I can go back to driving an internal combustion engine. Marketing folks need to stress these advantages.