After World War II, the victorious Allies were able to divide and decide the political fate of the Axis Powers and their conquered territories. This includes the USA drafting Japan's new constitution post-war, which Japan accepted with only minor revisions. In the 60-70 years since, the foreign-drafted constitution has seen very few changes.
One enormous change to Japan's political structure, as outlined by the new document, switched Japan to a parliamentary system while retaining the emperor. However, the emperor became merely a figurehead with no real power.
Also shockingly, the constitution forbid Japan to wage war, a provision that has stood unchallenged since its formation. The document detailed that "Japanese people forever renounce war as a sovereign right of the nation
and the threat or use of force as means of settling international
disputes," a seemingly permanent restriction on the right to wage war.
In a similar vein, the constitution stated that "land, sea, and air
forces, as well as other war potential, will never be maintained". This stood until 2005, where it was moderately amended to allow a defense force.
Most of the American bill of rights was transferred to the Japanese constitution as well, including 39 articles on "basic human liberties" such as adult suffrage, unions, and marriage and property rights for women. However, the UN has raised some complaints about the enforcing of these articles.
And how has such a restrictive document formed by a foreign power governed a developed country for so long? Despite Article 96's assertion that amendments can be added to any part of the constitution, the actual process is arduous. Two-thirds of each house of the Diet must approve of the proposed amendment before a referendum can be held. In that referendum, a majority needs to ratify the amendment before it can be added. This certainly contributed to how no amendments have been added since the constitution's conception, though existing amendments have been revised once or twice.
Time will only tell if this constitution continues to uphold Japan or if it will see further and more extreme revisions. The Japanese Liberal Democratic Party consistently lists constitution revision as part of its platform. For however the constitution was formed and for whatever reason it was formed, it has stood largely uncontested for several decades.
Sources: Wikipedia, PBS, Solon.org