I'm not a headquarters 'insider', but I have long time friends who are in position and are active in the expansion project. From what I understand the liquidation of the Brooklyn properties was being set in motion a decade ago - for multiple reasons. One was the impracticality of owning so many buildings throughout the borough. Another was the increasing costs and amount of time required to maintaining these properties/keeping them up to code.
So the reasoning was to move upstate, consolidate all the offices and residences and make something brand spanking new.
Why they chose to relocate to Warwick is something I don't have an answer to. In my mind it would have made much better economic sense to settle in the middle of the US - to be centrally located and where land is much more affordable.