It looks to me like there is more and more "stuff" (mechanical equipment etc) being crammed into the top section and this may be why they haven't done much to complete the "skin". It appeared almost empty when first constructed. Now it's not at all empty:
http://www.salesforcetower.com/cam
As I've said, I think the final facade panels can be put in place rather quickly when they are ready (and/or the panels are available it that's the hold-up).