I think it's a combination of factors:
1. Fear of reliability post-warranty (2 years)
2. Newer bikes are substantial upgrades re: features, power, range, refinement
3. Buyer market is still quite small. local buyers market is even smaller, unless you live in LA / SF / etc.
4. Where do you sell? ebay, cycletrader, local newspapers, online motorcycle forums?
5. Limited end-user access to the powertrain (see #1)
6. Significant rebates (incl state) allow owners to accept a lower price when selling. If your $14000 MSRP bike actually cost you $10000, then you can take a $8000 offer if it moves the bike fast.
For all of these reasons, a motivated seller will typically have to take a bath to sell, especially if they're selling after a year or two.
On the other hand.. buying a hugely depreciating asset new and then selling after a year or two is rarely a winning strategy. Gas bikes change so slowly that a two year old bike is likely identical to a factory new bike.
I would suggest the alternatives if low resale makes you leery:
1. Consider how long you really want to keep the bike. I really want ABS on my next bike, so I'm not swayed by the 2013 Zeros... much. I can ride mine until the 2014s or 2015s come out, then look at new and used bikes.
2. Consider a used bike. The 2012 bikes were excellent, but the 2013s are much nicer. However, a $9000 2012 Zero S ZF9 with a few thousand miles on it will run faster (top speed) and farther than (for example) a 2013 XU ZF5.7. If you have your heart set on a 2013 and new is too steep (even with the federal credit, which expires at the end of 2013) .. then the 2014 bikes will come out and you'll see a similar dip in 2013 used prices.