Wow, that's a rather broad question.

I think a lot of factors came into play, not least of which was sheer luck. The samnites were actually much more expansionistic then rome in the beginning.

The romans were rather lucky to beat them....once italy was under their control, it was probably similiar to Rome:Total War....something of a steamroller effect. Especially once Qarthadastim was gone....their was really no major power in the western med then.

Not to mention, in the east, there were several powerful empires (which you already know from playing EB, obviously), but they were constantly at each others throats, and Rome was free in the western med to do what she saw fit.

Once greece fell and pergamon volunteered itself, it was almost inevitable.

Not that taking gaul was easy by any standards.

They were lucky, very lucky.