Mary Beard just gave an interview on the NPR program Fresh Air last night. Her answer was basically "no bloody clue, but here's a guess."

Her argument was simplistic, but it was addressed to a lay audience. She guesses it had to do with Rome' almost unique custom of integrating former enemies into the Roman political system, either as citizen or some form of ally, and since Rome's allies provided much of the manpower for Rome's armies, the practice created positive feedback loop: As Rome expanded, it gained access to more man power, which allowed it to expand, wash and repeat.

There are obvious problems with this. She doesn't even try to explain why Rome developed this almost unique practice, which is really what we want. She supposes there must be some deep trauma in Rome's formative period that drove them to this inclusive model of citizenship.

The second problem is that Athens with the Athenian Empire, while not allowing the allies to become citizens, did practice some integration, such as unifying coinage, using allies in the military, and moving courts of law to Athens. So while Rome is unique, it wasn't quite as unique as Beard made out in the interview.

There is also the problem of the change of practice in the 3rd century. According to Roman legend, Rome relocated some communities forcibly to Rome (Alba Longa) and incorporated others as still independent allies, but after Punic 1 Rome dropped this practice and instead set up permanent provinces in Sicily and Sardina/Corsica. That's a very different sort of arrangement that Rome's treaty system, however oppressive Rome's treaties could be.