A new Leaf is $30k base. A car which depreciates that much is going to look really bad to new buyers versus one that doesn't. And without new buyers there are no used cars to sell at $8500. Tesla is affordable to something like 50% of new car buyers and if it sells for $20k more used then paying $10k more is an easy decision if you can afford it.
edit: Or in other words, used cars are irrelevant in terms of market adoption since their supply is limited by the supply of new cars. So you need to look at the demand and sales of new cars.