The reason you will never see any six-story buildings built is the building code. Four stories is the tallest you can legally build with wood framing; higher than that, you effectively need steel framing, and the code requirements for those don't make any economic sense until you get to 10 stories. In between is a dead zone where it is uneconomic to build.
Interesting to know! I'm OK with 10-storey buildings, I think we could build a ton of those in areas like SoMa without injuring the character of the city any.
