Tradition more than anything, I suppose. The propaganda at the time of Augustus' time of emperor was that he was the 'saviour' of the republic, not someone who had replaced it with an entirely new order. The senate did continue to exist even into Byzantine times, even though it was less powerful than before. Even when the character of the Empire became increasingly "autocratic", it probably didn't occur to anyone that the name ought to be changed.

Incidentally, SPQ "X" has been used by many European cities since the middle ages.