The issue comes down to new side-impact crash standards that require motor vehicles to better withstand a collision with a stationary object like a pole or a tree. The Vantage and DB9 models do not meet the new regulations, and Aston, it seems, doesn't have the wherewithal to re-engineer the cars to meet the regulations. But given the small nature of the independent automaker and the relatively small number of vehicles it sells, NHTSA has granted Aston an exemption.
As a result, instead of being forced to comply with the new regulations that took effect for the coupes this past September and for convertibles the next – or else withdraw from the market altogether – the DB9 coupe will have until August 2016 to comply, while the DB9 Volante and both coupe and convertible models in the Vantage line will have until August 2017.
It's entirely possible that, by that point, Aston will have all-new models on offer, potentially replacing the Vantage and DB9 models or giving it sufficient new products to offer that taking those older, non-compliant models off the market would not cause it the same degree of financial harm. The automaker has an all-new platform in the works and a new engine deal with Mercedes-AMG in place, and was recently spotted testing what could be the first of its new generation of models at the Nürburgring.