In 704, the era name in Japan changed from Taihō (大宝) to Keiun (慶雲) primarily due to the traditional practice of changing era names to mark significant events or to usher in good fortune and auspicious beginnings.

The Taihō era, which began in 701, was named following the enactment of the Taihō Code, a comprehensive set of laws and administrative reforms. However, in historical Japanese practice, era names were often changed after a few years, whether due to natural phenomena, political shifts, or cultural considerations.

The shift to the Keiun era name in 704 likely signified a desire for a fresh start or was related to an auspicious event or good omen. Keiun, which can be translated to "celebrated clouds" or "auspicious clouds," conveyed a sense of prosperity and hopeful new beginnings during Emperor Monmu's reign.

For more details on historical era name changes in Japan, you can refer to sources like academic journals or history websites specializing in Japanese history.