The major reason for this decision was because the weather in May was mild across the country, avoiding periods of cold, bad climate in many regions such as in Hokkaido and the Tohoku region.
北海道・東北をはじめ寒冷で気候の悪い地域の多い時期を避け、全国的に温暖な時期の5月にしたというのが大きな理由のひとつとされる。