Because they're the best planes available?
> or a Boeing branded Airbus with Boeing markup?
Because the Boeing markup is less than the US import tariff?
For the same reason people currently buy "American" autos (and planes) built from parts sourced around the world.
In American management culture, marketing is the knee jerk solution to any problem and the holy grail from which all revenue is gifted. They are true believers in the "magic" of being able to "game" people into making decisions based on something other than logic.
This same culture explains how/why the internet we have today is built around privacy invasion and (wait for it) ... marketing.
Bit like when the two planes fell out of the sky and the third world pilots were blamed / strongly hinted at as being inferior
American management style is to focus on marketing as the source of all revenue to be exalted. Engineering, design and manufacturing are expenses to be minimized and controlled.
Right now, Boeing management is looking hard to find reasonable manufacturing scapegoats and when they do, heads will roll. Clearly, management had no involvement.
Gives you that warm, fuzzy and safe feeling doesn't it?