NASA was unwilling to believe any discount offered for use of a refurbished spacecraft until they'd actually done it, so the bid was made on the basis of a new capsule for each mission.
(The human-rated Dragon is going to be a different version anyway, with altered docking support, much beefier propulsion, altered solar panels, and of course, internal controls and life support. Current plan is new spacecraft permission there, too, though, perhaps for the same reason.)
