walter,
HQNI had a limited number of Transport Helicopter assets and these were tasked by its own Air Tasking Cell.
As a helicopter cannot be in 2 places at once it would have made sense (from a tasking point of view) to utilise the Chinook to carry the large passenger load to a destination "out of Theatre", and retain "in Theatre" the 2 Puma's that had been originally tasked. This would enable them to retain as much flexibility as possible in providing responses to Security Incidents.
You have to remember that this flight was an HQNI generated task; not one that was initiated further up the Command chain.