I had a chance to hear a discussion with the Durham City Manager about this last year. Basically the cities that are most willing to work with Google (favorable laws/willingness to guarantee removing construction red tape/etc) have a huge leg up. There are strategic reasons too, but Google wants to come in, lay down a bunch of cables and move on to the next.

They have the leverage to ask cities to actively pull down barriers that other ISPs would face, and the cities willing to do that are the ones who get consideration.