The reason is that FictionWise (now Barnes and Noble) is still working on the reader software for EInk-based devices. Before FictionWise was acquired by B&N, both Astak and Foxit announced that their EInk-based devices would support eReader DRM. Astak is still saying this, see
Adobe Digital Editions for EZ Reader. We don't know what B&N's plans are, but it almost certainly includes their own brand of EInk-based device with eReader support. There may still be advantages to the Astak devices, since they will support both Adobe and eReader DRM.