The Technomage and Centauri novels are absolutely amazing. The Centauri trilogy is the REAL ending of the series. And the Mage novels portray the Shadow War in the big budget way they should have been.
Mages get their powers from Shadow Tech being implanted directly into their bodies. These crystalyses (sp?) were directly supplied to them by the Shadows and they have no idea how to replicate the technology. So when the Mages ran away from the Shadows in 2259, they lost the ability to gain new crystalses and make new Mages. Only the currently existing Mages can have powers, and when they die, the Mages will be extinct.
I'm guessing that at some point in Crusade, Galen would have discovered the means to replicate the crystalses, probably from the Earthforce Conspiracy's experiments, and thus make Dureena the first of the New Mages.