Let's consider why Airbus chose Toulouse and Hamburg for their final assembly lines.
Look at the location of Toulouse and Hamburg on a map of Europe--note Toulouse is close to a major shipping port (not to mention it has been historically home to the French aerospace industry since before World War II
), and Hamburg a major shipping port for Germany. As such, large components for Airbus airliners can be easily brought in by boat in addition to Airbus' in-house fleet of A300-600ST transports with oversized cargo carriage.
Boeing chose to build its its widebody airliner factory at Paine Field because it was very close to the Port of Everett, and even had a railroad line (one of the steepest in the USA in regular use) built from the port up the hill to Paine Field. This allows Boeing to have large subassemblies shipped in by boat, notably the Japanese-built subassemblies for the 767, 747-400 and 777 airliners. Given that the 7E7
will use the same Japanese subcontractors to build subassemblies for the plane, it would be easiest for Boeing to build the plane at Everett, especially now with so much unused production capacity available and with Boeing's stated goal of using even large subassemblies for the plane.