The city really had nothing with the actual placement, the head of RSA decided Mobile was on the rise and thought this would spur even more developments. It did and downtown is booming. The building was an investment more than anything.
If you don't know, RSA is short for Retirement Systems of Alabama and the head is Dr. David Bronner. You might know him when RSA held a majority in US Airways and he was the chairman. RSA owns the Robert Trent Jones Golf Trail in Alabama and various buildings around the US... most notably 55 Water Street in New York (among others).
Here's a link to some of their investments:
http://www.emporis.com/en/cd/cm/?id=retirementsystemsofalabama-montgomery-al-usa
Edit (Addition from Wikipedia):
"Since the beginning of the 21st Century, RSA has been ranked among the 20 largest internally-funded pension funds in the world."
By the way, since you're from ACC country you might know Raycom Sports. RSA owns part if not all of that.