Well apart from the much longer runway than Carlisle that can handle jet aircraft of a reasonable size (B757, B738) and as it has a history of handling them there is less likely to be the opposition seen at Carlisle
Much closer to a major population centre
Closer to their major hub at Warrington
Closer by a bit to other transport routes
Oh and longer declared runway distances than Carlisle & Southend
But they already own CAX, have spent a shedload of money on it and have planning permission for their freight depot
Oh, and your assuming they want CAX as an airport - why do they want/need an airport at Blackpool?