The more north you go in downtown, the less the FAA cares.
It would have been much more expensive for State Farm to build an 800 footer downtown and come up with a parking solution for it if if they wanted to build there. The Devon tower has about a third the number of parking spaces per square foot than the Tempe project.
If the 8,500 spaces in the State Farm Tempe complex were under one roof, it would rank as one of the 10 world's largest garages as of 2008.
http://www.forbes.com/2008/04/10/par..._slide_10.html
The circulation for that would be an absolute nightmare to handle appropriately and probably be incompatible with any of the street layouts downtown or in the vicinity, if even such a monstrosity could be built in the central city without raising ten thousand eyebrows and complaints.
Given that offices coming online today require far more parking per square foot than they ever did, I actually don't see Phoenix getting a new tallest ever, even if the economy were any good, and similarly I don't see the vacancy rates in all the obsolete Midtown buildings ever significantly improving as well unless there's some major new investment into them.