The simple answer? It's Japanese.
The convertible was based off a production car that numbered in the hundreds of thousands. The Deloreon was based off no other car and looked like no other car on the road. Also, it was a small company that only made one car. And when they originally sold, sold for a lot of money.
Seriously. Most of the cars were bought by well to do middle aged men for their mistresses or trophy wives to drive around. They saved the BMWs for their daughters.
I suppose if some head exec of Nissan in charge of the project could have gotten himself caught on cocaine or other drug charges relating to the development of the convertible, the value might go up.
But the simplest answer is, it's a cheap Japanese car that surrounded no mystique or aura except to a few people who own one.
BTW: Did you know Deloreon is still selling cars?
http://www.delorean.com/remanufactured.asp