I've noticed that they seem to be clustered in certain states. Certain markets. Northwest, Idaho, Pennsylvania, and Georgia.
I can think of a few reasons why this is the case. One reason is cost. New York, Los Angeles, and the Silicon Valley are not in the list of those cities. Those are cities with high densities and likely greatest demands in terms of population. The initial deployment costs will be huge.
The biggest cities with WiMax so far are Las Vegas, Philly, and Atlanta. The next major cities to be launched are Chicago, San Antonio, and Chicago. The cities are bigger this second time around. In the year it took for the WiMax network to be built out, cost have gone down quite a bit.
The second reason why Sprint brought WiMax to small to mid-sized cities is so that Sprint can more properly gauge interest and test the network. The cost will be less than trying to start building out the network in major US city like New York.
Also, by building small to start with, any issues can be isolated, tested, and rectified more easily. Imagine trying to do that in a city the size of Los Angeles and as spread out. And then imagine all the complaints and bad press that's likely to result from it.
My fear is that Verizon Wireless and ATT will likely follow this pattern. However, given the pace of competition, and anywhere between $30-40 billion being spent on wireless networks, the LTE network will launch with more cities than when WiMax started with.