Because, between the two of them, they carry more than half the traffic between the eastern US and London. That caused anti-trust concerns in the US and British governments.

These concerns are also the main reason that members of AA's and BA's frequent flyer programs can't earn miles on the other's trans-Atlantic flights, even though they earn full miles on all their flights on any other routes (AA members get miles for BA flights onward from London, BA members get miles for AA flights once they reach North America) and on the other oneWorld partners' trans-Atlantic flights.