I would imagine when you start talking about luxury cars automaker isn't so relevent... the market for a vehicle like the Q45 is limited, and most of the people who would look at it are looking brand new.
People who have $35 000 to spend on a car are probably still at the point where they want something fuel efficient, which the Q45 most certainly is not. Cars like the Civic et al. hold onto their value so well because they're bullet-proof, economical, and desirable, and practical.
The Q45 isn't as desirable (high fuel costs) nor as practical. So as the market shrinks, it's bringing less money. Also, Nissan/Infinti is not at the same point as Toyota/Lexus in terms of brand perception (imo).
All that said, that's a helluva deal for a very nice car. If I was looking for a brand new luxury sedan, I would seriously consider that (go used) and a nice little sports car to match...