because for a long time the romans had the best army in their area. They were the leading power in training, strategy and also in technology. One reason for this is that rome was the first really centralized state in europe. Many territories were lorded over by local tribes that fought against each other and did not unite against the romans. Often it was quite enough that the roman army marched somewhere and the local tribes agreed to become part of the empire - because the romans in general had a policy of religious tolerance, to local kings remaind in power their kingdoms were protected by the roman army,... in return they had to pay taxes and to make sure there would be no upraising against roman superpower. (this was the case with the jews so after a few gernations of constant guerilla warfare the romans destroyed jerusalem and sent the people into diaspora).
Hope this Helps!! :))
