That’s a good question. Perhaps it’s a matter of frequency - up there, are you practically guaranteed to have ice each winter? If so, then I can certainly see why the lines are buried. Here, the odds of having a tornado or wind storm are small - well, smaller than the chance of ice in Minnesota - therefore the less expensive option is to hang the lines on poles. I’d wager that the power company has studied the economics of it already, and does the most cost-saving thing depending upon the situation.
This is gradually turning around. As new commuter lines are being built into Tokyo (the Tokyo Tsukuba express and the new Toneri Line from Nippori), new neighborhoods are sprouting up around each station, and this time around there seems to be more urban planning going into them. The streets are generally wider, there are actually sidewalks, there are more parks and greenspace everywhere, and the utility lines are all underground.
As an example, contrast the newer east side of Minami Senju station:
with the older west side:
http://www.flickr.com/photos/jmrocher2001/1992957616/in/set-72157594557423470/