Yep, though there was also some ideological thinking behind it. To the Romans, the essential point of the Res Publica was that it was for the good of the public - hence the name. The Emperor, no matter how exalted, was still only a magistrate - everything he did, theoretically, was in the name of the power invested in him by the people. A monarch held power in their own right, in the same way that a citizen held power over their own property; the Emperor had no such (theoretical) power.

Funny enough, this would be the same basic reasoning used some 1800 years later by Napoleon when proclaiming himself Emperor of the French after a (dubious) plebiscite.