Carthage fell to the might of Rome, I think it was, three times in total. They were completely defeated and accepted some pretty horrible terms of surrender. And yet they maintained their self sovereignty until they were utterly destroyed at the end of the third Punic wars. In most other cases I know of, Rome absorbed the nations they conquered. They become provinces and citizens of Rome.
Particularly the second and third time Rome defeated them they had serious fears of Carthage raising again, so stationing a permanent Roman presence would seem logical.
So why was it that the city of Carthage was not converted into a Roman city, at the end of the first or second Punic wars, or as an alternative to the destruction that happened at the end of the third Punic war?