Japan actually partially democratized during the Taisho Period between 1912 and 1926. Also, Japan was historically a constitutional monarchy with claims of there being a constitution as far back as 604 and with the Charter Oath of the Meiji Restoration, so I wouldn't fully support the claim that Japan had a long history of "autocracy" as it actually had a constitutional background. Finally, US occupation for seven years can do a lot to support the development of democratic institutions (or at least it used to be able to).

However, we should also remember that the LDP has basically run Japan for its entire democratic history with a few intermittent period when the opposition was in government. It hasn't been a one party state like Taiwan was or a dictatorship like South Korea was intermittently as there were always opposition parties, but they have mostly been ineffective. Even today, the opposition parties are best known for being against the government rather than for something.