Because there were not than many sold, and, presumably owners are happy with them. The ones that are available command a significant premium as they're young and perceived as very fuel efficient.
Things like this make me laugh; http://www.approved.bmw.co.uk/bmwuk/bmwauc/details/0,,1156___ask-16%40adm-xx,00.html?ch=_V0JBTEw1MjA5MEU3NTEwOTQ=&btp=1x24-distance_ASC|list
Because you can get this car new for nearly that. Sure add on the goodies and it goes up, but, the APR on AUC is much higher and no discount on AUC whereas new you're still able to get thousands off. Why would you buy the AUC. You wouldn't.