This is because this position was 'a government position that should be assumed by a Prince' ("Gyokuyo" - Diary of Kanezane KUJO), and it was customary that the candidate become Gonnosochi (Provisional Governor-General) or Daini (perhaps because of complications, in the end he settled for the position of Dazai gon no sochi - Provisional Governor-General of the Dazai-fu offices).
正帥は「親王任ずる所の官」（『玉葉』）で権帥・大弐になることが慣例だったためである（紛糾したためか、結局は大宰権帥となることで落ち着いている）。