Explanation: When Rome began to be ruled by Emperors, not only did they still regard themselves as a republic, but they remained ardently anti-monarchist (with regards to who ruled them - they were fine with other countries having kings).
He's JUST the first citizen!
Funny enough, Greek cities would often write to the Emperor calling him 'basileus' ('king') during the first ~300 years of the Empire - whereas the imperial secretaries would write back, thank them for their concerns or whatever they were writing about, but insistently in the name of simply 'Caesar' or the 'Autokrator' ('self-ruler', 'autocrat', or 'dictator', as one prefers).
NOT A KING
NOT A KING