^^In all seriousness and with some regret, it's mostly NIMBYism. Everywhere you look, from the Third St. corridor to which you refer ("south of the Bay Bridge") to transit-oriented developments in the East Bay, they are not taller because community members successfully fought to keep them from being taller. Specifically on Third St., I can recall when UCSF wanted to build a dormitory tower in Mission Bay and the Potrero Hill "acitivists" convinced then State Senator John Burton to block it (
It's a bird, it's a plane, it's Super Burton laying UC low ).
The people living in San Francisco's low-rise, historical neighborhoods like the ambience of their Victorian villages and they like their views to the water and beyond (which often have real value when the time comes to sell) and they will fight bitterly to have nothing blocked even though, time and again, the courts have said that individuals have no legal right to an existing view.