Kings, Consuls, Decemvirs, Dictators, and Triumvirs, were all in the past in John's day.

Emperors were then ruling the Roman world; and the empire was still pagan.

Six of these heads, therefore, Kings, Consuls, Decemvirs, Dictators, Triumvirs, and Emperors belonged to the dragon; for they all existed while Rome was pagan: and it was no one of these that was wounded to death; for had it been, John would have said, I saw one of the heads of the dragon wounded to death.
The wound was inflicted after the empire had so changed in respect to its religion that it became necessary to represent it by the leopard beast.

But the beast had only seven heads, and if six of them pertain to the dragon, only one remained to have an existence after this change in the empire took place.

After the Emperors, the sixth and last head that existed in Rome in its dragonic form, came the Popes, the only head that existed after the empire had nominally become Christian.