The explanation must be based on work that someone in BA's O.R. team (one of the best in the U.K.) has done using passenger data and cost-benefit analysis. Two A380 planes would carry the same number of passengers as three conventional planes used on this route. That would be cheaper, and would give BA another slot for landing at Heathrow Airport. So why stick with the current timetable and allocation of planes? Answer: "Revenue".
Many customers on that transatlantic route are making the journey on business; they are not transit passengers at either end; they are paying full fares, and for these customers, frequency of flights is of very high priority. So BA is likely to stick with the current number of flights, to squeeze as much revenue from the business passengers as possible, by offering them a frequent service. A significant number of these passengers will pay for a flexible ticket so that they can turn up and go when their meetings are over. (Having worked in academia, I have never been in such a position; my tickets and times have been fixed weeks before.)
But other airlines may reach a different conclusion. Passengers between Paris and New York are less likely to be business people (language) and more likely to be transit passengers at one of the airports. Similarly for Frankfurt to New York. So the French and German flights may use A380 planes for the transatlantic route. And BA is likely to use the plane type for flights to the American west coast, where the frequency of flights is less of an issue.
|A380 in BA livery (c) xfwspot|