The Meiji Constitution in 1889 set up a new Japanese government along parliamentary lines. The leaders of the Meiji restoration were highly influenced by Britain and Germany and thus went with a form of government based on both countries.

The US in 1946 did indeed have the ability to remake the Japanese government structure as they saw fit. The Japanese initially proposed a new constitution that was essentially a lightly revised version of the 1889 one, but that was rejected by SCAP (Supreme Commander of the Allied Powers - MacArthur's office). The eventual constitution that is in place to this day was written mostly by Americans. So why isn't it similar to our own?

The answer is that that the committee set up to draft the constitution was did not want to present something totally foreign to the Japanese. It had to be more liberal than the rather un-democratic 1889 constitution, but if it had no resemblance to Japanese political thought then the long-term stability of the government it created could be at risk.

Those working on the problem had to bear in mind constantly that they were dealing not only with the abstract problem of writing a democratic constitution but that they were proposing the most far-reaching political reforms for an entire nation, and that, to the greatest extent feasible, the provisions and institutions should be designed to fit the Japanese political picture. The document was to serve as a model or pattern, was to be presented as a statement of general principles but in such form that their application could be readily understood. Unless, however, the statement bore some relationship to the existing state of things, it was obvious that it would not serve its purpose.

MacArthur himself highlighted the risks of a constitution that was too drastically unfamiliar earlier during the occupation:

He said that it was his hope that whatever might be done about constitutional reform would be done in such a way as to permit the Japanese to look upon the resulting document as a Japanese product, for he believed that only in this way could the work be permanent. He said that it was his conviction that a constitution, no matter how good, forced upon the Japanese by bayonets would last just as long as bayonets were present, and that he was certain that the moment force was withdrawn and the Japanese were left to their own devices they would get rid of that constitution, merely for the purpose of asserting and maintaining their independence of ideas that they had been forced to accept.

The constitution was also drafted on a short timetable, meaning that keeping things relatively simple was a bonus.

While we can't know how Japan would have developed under an American-style constitution after the war, I do think it was wise of SCAP to write a constitution that was substantially more liberal than the one that came before it while being familiar enough for Japanese leaders to accept.

Quotations are from: "The Origins of the Present Japanese Constitution" by Robert E. Ward, published in The American Political Science Review.