Why would they? They had already conquered Carthage and broken it's power. They did not allow them to have a navy or maintain an army. They were effectively neutered. However, the Romans were not particularly averse to them continuing their commercial operations so long as they were not a threat. However, when the Carthaginians suddenly became a threat again there was a feeling that they had had the opportunity to become a Roman client but had refused and thus the Romans were justified in destroying them. utterly.

It should also be noted that the city was mostly burnt to the ground during it's capture anyway, and Scipio Aemelianus was very upset about it, Polybius tells us that he openly wept. That did not stop him from burning the rest of the city to the ground and selling what remained of the population into slavery though. The story about him sowing the fields of Carthage with salt so nothing would grow there is almost certainly a fabrication.