If you think about it, WW2 only happened 70 years ago. In city development, that is not a large amount of time. One of the reasons Tokyo was able to modernize so quickly was because of the bombing during WW2, done mostly by the US. The damage done on Tokyo was not quite as extensive as the bombing on Hiroshima and Nagasaki (where we dropped nuclear bombs), but it still destroyed a large part of the city.
One of the positive things that came out of the war for Japan, was the chance to modernize. After the war ended, the US forced Japan into a trade agreement, where the Japanese would sign a treaty, and the US would help give supplies and money to rebuild. When they built most of the building were modern for that time period. As time went on Tokyocontinued to develop and modernize just like everyone else, only they were a little ahead.