Why is the Sakura Wars theme song so famous in Japan?

The song is Geki! Teikoku Kagekidan, the theme of the Imperial Combat Revue. Lyrics are by the series' creator, Oji Hiroi, who spent a month or two learning to write song lyrics for the game; the music is by Kohei Tanaka, for whom Sakura Wars was the first major work in games and the score that brought him widespread recognition. Hiroi's brief to Tanaka is documented and wonderfully specific: combine the music of a Super Sentai opening theme, the squadron hero shows every Japanese child knew, with the vocal style of the title song from the 1949 film Aoi Sanmyaku. Tanaka composed it and got it approved in a very short time, and it was sung by Chisa Yokoyama, the voice of heroine Sakura Shinguji, with the rest of the cast.

The aim was bigger than one game. Tanaka and Hiroi both held that rhythm and percussion dominated Japanese popular music at the time, and they wanted to reintroduce younger Japanese listeners to beautiful melodies. Every song in the game began the same way: Hiroi wrote the lyrics first and handed them to Tanaka to polish and set. The result is a theme that behaves like a folk song by design, bright, marching, and easy to carry out of the theater in your head, which in Japan it promptly was: the song is known there by the short nickname Gekitei, and its reputation runs ahead of the games themselves.

Sega has treated it as an heirloom ever since. Revised versions opened Sakura Wars 2 and Sakura Wars 4, and when the series returned with the 2019 reboot, Tanaka and Hiroi made Geki! Teikoku Kagekidan: Shinsho, a new chapter of the same song. That time the process reversed for the first time: Tanaka composed the music first, and Hiroi wrote the lyrics to fit it. A theme song commissioned to revive old fashioned melody ended up outliving the console it was written for.