IIRC there was some concern that HQ2 should be on American soil or withstand the backlash of the current political climate.
Big companies are funny animals in that they appear on the surface to make short-term decisions to appease the market, but behind the scenes tend to plan out a decade or more. A good fifteen or more years before Boeing moved their HQ out of the Seattle area and moved significant production elsewhere, they were putting out messages to suburban governments where they had smaller plants to not count on them being around. I would bet Amazon or any other company thinking of a move of this magnitude has a plan that stretches well beyond 2020.