I feel that UA have fallen far short in their marketing, not that it was ever to be an easy task, EK were much more slick, in that it was much easier to bring NCL within their national advertising campaigns with one destination (& expanding hub) similar aircraft offered nationally (at least pre A380) & a daily year round service, irrespective whether MAN/GLA/BHX etc i.e a more or less homogeneous product providing greater accessibility for onward travel.
Then there is the other problem that one can not under estimate FF programs & customer loyalty. Most people I deal with wouldn't have a clue as to what aircraft they are to fly on but are acutely aware of the airline so UA probably don't score too low on the 757.
What they possibly may find is that historically UA was never a code share that was ever much use to get to NCL when considering booking engines, ironically it was not such a problem to get to DTV when BMI flew DTV-LHR .
I sometimes wonder that if AA had taken on the route this year instead would they have been able to generate more pax traffic with their BA alliance.