This a great question and there are many causes that one can point to. I will list the three most important.
First, the Romans had an uncanny ability to learn from others. So, they took the best from all the people around them. They learned to build ships from the Carthaginians and took religious principles from the Etruscans and learned about law through the Greeks. So, they had the ability to assimilate others.
Second, they were great warriors. They were disciplined and fought well together. Their military was unmatched and this went a long way.
Third, they had a form of government, which was competitive in nature. So, this caused them to seek glory in victory home and abroad. This culture lends itself to power.