The "low" resale value can be
The "low" resale value can be primarily attributed to the incentives you mention.
When shopping in Feb 2015, I found that the after-incentive price in Texas was $21,000 to purchase a new 2015 Leaf, $19,000 total payments to lease a new 2015 Leaf, or $14,000 to buy a used 2012 Leaf. This is a 33% depreciation after 3 years.
According to Edmunds, the average depreciation for a gasoline car is 46% after 3 years.
Thus, the "high" rate of depreciation is mostly driven by the auto industry's determination to ignore the simple reality that buyers factor the incentives into their valuation of a used car - exactly as I did.