Training is a specialized position, and training Captains are no exception.
Several reasons...
Rapid planned fleet expansion, which requires additional trainers, problems with recent upgraded guys so the airline management wants those from outside to have an unbiased look, lack of suitable folks from line flying, etc.
Direct entry folks with the requisite experience will always find a position, whether the present line folks like it or not...and from some of the comments here, some don't.
