Sure, but the density of, say, NYC, Chicago, or SF, is pretty high. If density were the only factor, parts of the US would rank much more highly than they do now, would they not?
American Fiber is a corperate venture and doing work in the large cities can be expensive (especially New York) so maybe that has something to do with it.
I'm guessing doing stuff in London is pretty expensive too, so I doubt that reason is valid.