The population density factor in big cities allows a rapid return on investment for the carriers who then systematically roll it out into the rural areas. But without the ROI in the big cities you then are in the same situation as the US. Other thing is the countries overall scale (on a physical level) to quote myself "Japan is 2/3 the size of California with 45% of the population of the entire US."

THe cost on this is actually pretty simple. I have been living in Japan for 10 years and yes we do enjoy some really incredible bandwidth here. Most of the population lives in very condensed areas. Greater Tokyo has about 30 million people in an area the size of LA... so rolling out the latest technology in one of the most wealthy and densely populated cities in the world is well... nearly easy if you can say that. Cell phones are the same way. Docomo, Softbank, AU etc.. rolled 3g out YEARS... before the US, simple put because logistically they can. Japan is 2/3 the size of California with 45% of the population of the entire US. 80% of the country is mountainous (ie.. nobody lives there) and half the countries population is centered in 4 or 5 cities. Tokyo, Osaka, Nagoya, Sapporo.. Heaven for Technology fans. In a nutshell, you can roll out new technology fast and cheap because the distances between hubs are short, and the overall physical breadth and width of the network is small.