Scott Galloway was pretty confident that it was always intended to be in Washington, partially because of Jeff Bezos already owning a house there: https://www.youtube.com/watch?v=_HyiY_m_YxI&t=542
I recommend watching the actual talk, but there are a few sections of it summarized (including some about Amazon HQ2) in this article: https://www.recode.net/2018/9/19/17878766/scott-galloway-pre...