I think there is a major bias against the towers from the finance/insurance industries in NYC. Too many of their colleagues and friends were killed or impacted the last time around, like working in a graveyard. Here is a harrowing story from someone who was working there that day:

http://www.wernerf.com/article/kamran.html

It is plausible that tech firms would fill in the gap as they don't have the same associations, the buildings are brand new and they can build out or subdivide as they see fit. Without huge demand for the space, I'm sure the tech firms are getting awesome deals, well south of the 50-60 per sqft quoted here when all incentives are factored in.

I'd even guess this is an example of Paul Graham's "The Submarine" in action - PR people trying to hype up demand for the building.