The NASA contract for the CRS missions specified a new Dragon for each flight.
That would explain why no Dragon has been reused on an ISS logistics mission.
But that does not answer why SpaceX has not reflown any of the others, which they retain ownership of, post flight.
SpaceX has proposed a commercial free flyer, where a Dragon would be launched with experiments and called DragonLab, reside in orbit some period of time, then return. The current launch manifest for SpaceX shows two such missions on the list, but without dates.
I heard Max Vozhoff on the SpaceShow talk about plans for DragonLab. They had suggested offering the Department of Ed, an option to buy an entire Dragon flight, and offer a competition for students from all states to supply experiments. But that never seems to have materialized.
It has been reported that components of Dragon's have been reflown, but not specifics of which components.
The CRS-11 missions in 2017 and pretty much all future Dragon Cargo are expected to be the first to refly a Dragon to the ISS.
Additionally the first few Dragon attempts were learning experiences, and changes were made based on lessons learned. There was a leaking problem with water after landing in one of the Dragons, and changes to the power/refrigeration systems in later models. It may be they need to stabilize on a design as mainly static before reusing.
Dragon V2 will probably be a different story, being planned for reuse from the start, based on many previous Dragon cargo flights and years of testing and upgrading.
As of Jun 2018, 3 Dragons have been reflown with number 4 due at the end of the month. CRS-11 and CRS-13 used the same booster (Core B1035). CRS-12 and CRS-14 used the same booster (Core B1039).
CRS-15 is reusing the booster (Core B1045, last Block 4) from the TESS mission.
Reused capsules on reused boosters oh my!