All any of us can do is speculate since WT is not transparent when it comes to its finances or reasons for doing things.
The Warwick land was a vacant contaminated property which they were likely able to buy cheap. Cleanup is normally costly but there are many JWs willing to volunteer labor so the costs may not be all that high compared to hiring a specialized contractor. Perhaps there are government grants available to offset some of the cleanup costs.
Although the new buildings cover a large area, again, they were built primarily with 'slave' labor. Building costs at least in this area, often equate to 50% labor, 50% materials.
New buildings are typically more energy efficient so it is likely WT's ongoing energy costs will be substantially reduced.
With regard to 9/11, WT may have received some bad press for locking their doors and not assisting evacuees on Sept 11.
I understand Brooklyn property owners were increasingly complaining over WT owning so much tax exempt property. Other property owners are then forced to pay more than their fair share of property taxes to offset those not paid by WT. I think the most complaints were not regarding places of worship, but more due to non-worship properties such as those used for printing, warehousing, equipment maintenance, parking facilities and residences for WT 'staff'.
Since I assume many of these complaints were made to the city, perhaps the city exerted some pressure or provided some incentive for WT to relocate. If the city didn't place pressure on them, perhaps it had become an expectation.
With WT cutting back on printing and in reducing staff, the buildings in Brooklyn would likely have required extensive alterations. They also would no longer require easy access for receiving paper or for shipping physical literature since the shift to electronic publications can be sent over a digital pipeline from virtually anywhere in the world.