That's because of supply and demand, and human nature.
In California, people are grabbing Volts for the HOV lane pass. And in some states (Colorado and West Virginia come to mind) there are huge tax credits in addition to the federal credit, so there is heavy demand from those states as well.
So when someone buys their Volt at, say, $32,000 after incentives (perhaps $30,000 before sales tax/fees), and decides shortly after to sell it, they are going to want to try to get most of their money back, so they may try selling it for what they paid ($32,000 in their mind). It's now $5,500 less than what they paid to the dealer.
There's also a few other factors at play, such as people who cannot use the $7,500 tax credit. If you could buy a new Volt for $37,500 (plus tax/fees), and know you didn't get a $7,500 tax credit, or pay $32,000 for a new Volt (plus tax/fees), which would you do? Some people would simply not buy a Volt because it doesn't seem fair to not get the full credit, but others would.
Further, there's the $2K incentives that dealers have been getting this year. These make it harder to figure things out (e.g. is the $5K discount below MSRP, invoice, or the ~$2K below invoice that some pay)? Hard to figure out is something dealers love! I'd say well over 90% or so of the "Is this a good deal?" posts in the Buying/Leasing section are about leasing.
As far as the 18-month old question about tax incentives for used Volts, to me that seems like "double tax-crediting", which doesn't make much sense to me. The $7,500 tax credit has proven to automatically pass down (in the $5k case, $2.5K for the original buyer and $5K for the second buyer). That's something that we didn't really know much about 18 months ago as the first Volts were only a couple months old (and nearly impossible to find).