[miaht82]

It all has to do with land availabilty. World Savings/Wachovia, Valero, USAA, NuStar, Tesoro, WaMu, all have certainly more than enough of a presence and # of employees to need space for tall, but why build up when you can build out in the outskirts (near your employees) for much cheaper.
I only really have D.C. to compare it to. D.C. has a large government and tourism presence, and most of the major corporate presence were in McLean,VA, but I would say that Downtown DC was great in the neighboring areas right outside of the "downtown" area.
River north will hopefully create the "infill" that is needed to help out neighboring areas and connect downtown to the rest of SA. San Pedro, Main St, St. Mary's St. and Broadway are all going to benefit from the expansion of the River.
We will probably see more residential high-rise or hotel before we see another office building go up. Once(/hopefully) the young professionals live downtown, we might see another tall go up. Maybe 15+ years?
We can't even get a good stretch of downtown with restaurants and retail in the immediate downtown area that doesn't have the riverwalk as a sidewalk and entrance.