Taxation was one of the major factors. Empires need funding. By collecting taxes from citizens, Rome was able to expand the goverment and thus expand the empire. Elected rule also contributed, because communities could have local representatives.
As previously mentioned, this question can be answered by refering to the geographical factors that led to the development of Rome. Rome's position on the coast of the Mediterranean meant that it was a key trading city which had easy access to other parts of North Africa, Asia and Europe, so that its development was in part due to its success as a trading city.
The early economy was based on small farms that were supplanted by larger estates as conquests of other groups were successfully undertaken. The economy took another turn when these continuing conquests brought in captives who comprised slave labor, thereby reducing the cost of economic productivity.