The short answer is: it's much easier to build in a greenfield environment where there is a "helpful" local government, ready access to rights-of-way, and underserved population, easy construction costs, access to labor, favorable tax climate, and a host of other reasons that come before "lots of people who want gigabit speeds".
The economics of building (or overbuilding) a gigabit fiber network drive you towards areas where you can get a high degree of penetration for your investment. That's not going to happen in a place like NYC or SF. The construction costs (directional boring) and tax regime (network cables are "property" for property tax purposes) mean that SF and NYC are probably going to be the LAST places to get Google Fiber.
