In both canons, the absence of the Empire’s civilian government in stories set after ROTJ. Assassinating a head of state doesn’t cause the civilian government to spontaneously be replaced by a military junta.
Well, Palpatine got rid of the imperial senate during the events of ANH and gave power to all of the Moffs (regional governors), who were the ones who started fighting each other after ROTJ.
Thrawn was an Admiral, not a Moff.