The cost of owning and maintaining the 330's is cheaper. Is it true that some 330's were handed to QF as compensation for the 380 saga? If so, the cost of owning them would be bugger all.
As for 20 year old 767's with high maintenance costs, that would not fit the LCC model, would it? Mainline suffers not just the increased costs of operating them, but also the loss of premium revenue from punters shafted with an older, less luxurious type.