This is because the present and former Imperial House Act didn't provide the rule for the abdication, and stated that the Koshi (Crown Prince) would be enthroned after the Hogyo of Tenno.
これは現旧の皇室典範が退位に関する規定を設けず、天皇の崩御（死去）によって皇嗣が即位すると定めたためである。