A330-300 has the lowest seat mile costs for the Atlantic. 777 is too capable, read heavy, for pure efficiency across the pond.

You don't see many A330's crossing the Pacific. That's where the 777 shines. Maybe 3-4 hrs more range vs the 330-300.

321 vs 757 cockpit costs? $30/hr less.
321 vs 757 total hourly cost? 30%/$1500 an hour less.

Net? They didn't buy 321's to save $30 an hour on crew costs.