Well, I think we're looking at a number of factors here. For starters, Japanese democracy dates back to the Meiji reforms and, although it was never particularly liberal to start with and certainly suffered during the Taisho era, it did leave the post-war politicians with a history and framework to draw upon. In a sense, it'd be more accurate to say that the existing Japanese democracy was demilitarized and given a new, more liberal constitution. This combined with Japanese economic strength, disillusion with the war time government, and the continuity of maintaining the emperor helped ease the process considerably.

I think there's also an argument to be made that the close proximity of soon-to-be communist china and the still ongoing dispute over Russian (then soviet) possession of northern islands served as an incentive to try to rebuild under the protection of the American military umbrella.

Without going too deep into a modern comparison, any chance of a religiously motivated guerrilla resistance was undercut strongly by the imperial announcement of non-divinity.

In any case, democratization was by no means uninterrupted. Strong showings by the socialist party in early elections alarmed both conservative Japanese and the American establishment, and as late as the seventies you have people like Mishima Yukio calling for the rule by the emperor (although by that time, he was a bit of joke/anachronism). There was a deep period of unrest among students in the 60s, but obviously nothing that would compare with the countries you mentioned.