The ocean of concrete around a typical suburban office building is very expensive to remediate. Downtown buildings often have integrated parking garages which create substantial expense when converting to residential. Also, family residences may actually increase parking demands in downtown properties (i.e. walk/train/etc to work means less demand for spaces then when 2 car families move in). If converting to residential was doable outside of extremely low value/tax incentive properties, every REIT would be converting their poor performing commercial holdings... It's just not simple, easy or rewarding right now.