The big thing is that transfer pax at MAN would likely be only able to fly to an AA hub in the US (if going to the US) and be forced to connect there, since there will be insufficient demand for all but a few destinations. You'd have to price these two-connection routes cheaper that the LHR routes that allow one to fly CPH-LHR-wherever, and then you're largely carrying leisure passengers and losing out on high revenue business travellers.