Why does Polybius think Rome is superior to Carthage

Polybius wrote after Rome had defeated Carthage in each of three wars over a period of 120 years, after the third one destroying the city and selling it people into slavery. He would think Rome superior, wouldn’t he?

Polybius did not just think that Romans were superior militarily. He also thought that they were superior in their political institutions was well, and that this accounted for their military might. He thought that the Romans were superior to the Greeks as well as the Carthaginians.
Polybius was one of the Greek hostages taken by the Romans following the 3rd Macedonian War in Greece. He spent 17 years in Rome. He was treated well and respected. Aemilius Paullus the Roman general who won the war and took the hostages entrusted him with the education of his sons. The Roman elites admired his intellect and often asked him for advice, even on matters of war. He was given access to archives for his studies.
Polybius became a supporter of Rome. The aim of some of his writing was to explain Roman tradition and portray Rome in a good light to his Greek readers and make them more favourable to Roman rule there. When he went back to Greece he helped the Romans in their relations with the Greeks.
Book 6 of Polybius’ Histories is an analysis of the Roman constitution. In fragment 7 he argued that Rome was superior to the Greek states of Sparta, Crete, and Mantinea and to Carthage. His writing of the superiority of the Romans over the Carthaginians is very biased, has a strong propaganda flavour and facts are presented in a far from judicious manner.