There are also economic reasons. Parisian density is only possible with a great underground transportation system. That's hugely expensive to build and takes decades. SF has traditionally been a boomtown economy with corresponding bust cycles, so it's harder to make those kinds of investments. They are happening (e.g. the Chinatown extension) but more conservatively.
Saigon where I'm back from is building a light rail to the center with twenty odd 30-40 story residential blocks along it. The whole thing is taking about 2 years to build.
That kind of thing is good for preserving historic buildings too - the blocks are all in nondescript areas 1-4 miles from the center.
