Because lots of types of work that aren't development don't fit well with the remote team model. And HQ2- just like HQ1 and plenty of other Amazon offices- won't be just developers.
When I worked in the Seattle office, the floor below me was some kind of fashion department. I don't know what specifically they did, but the ones I met weren't writing code. In the Toronto office today, there's an entire floor that are AWS Sales Evangelists (or something like that). Business teams like that can't always work in a purely remote environment.