The last part of OP's statement is the key. In a field that's rapidly advancing technologically, used prices are depressed because the new product is that much better than the used product.
Think back to the early smartphone days - every year phones multiplied in performance, in screen resolution, etc. In that environment a used item is less attractive because you feel like you're missing out on features/capability. This keeps used prices down. Nowadays used smartphones are more competitive because the rate of advancement (that buyers care about at least) has slowed.
For example there's another post later in this thread that points out that the Nissan Leaf has been the same price forever - except the current-gen Leaf has literally double the range of the last one. Effects like this depress used prices.