Has anyone noticed the new JetBlue A321 has not moved in over a week? flightaware.com/live/flight/N903JB
Or How about the Delta 737-900ER?
flightaware.com/live/flight/N801DZ
Anybody know why? 
Well, for the A321, they need to get a certain number in the fleet before they can start operating them in revenue service since they need some semblance of fleet consistency on the routes it will serve. There’s no sense in flying it around for no reason until it enters revenue service.