There's a lot of factors since the question is too general, but I think I understand what do you mean. On several levels (philosophy, theology, literature, arts, etc.), I agree here, they achieved more complexity or a deeper approach than the northerners, but I can't be sure as a whole; as I said, we're missing too many vectors here. I'll try to give you a general answer to a general question, though.
I'd say that the main factor was the commerce. At that time, the Mediterranean sea was the perfect place to spread an economical red, being greeks and phoenicians the better examples of this early impulse, which benefited these peoples in two senses:
To form commercial colonies (directly or indirectly) that allowed a better functioning of the economy of the Metropoli. Thus, a better quality of life, the flourish of the arts and the spread of a concrete culture were more feasible.
And consequence of the first, to allow a common knowledge between different cultures. For example, we know that the communication between the members of the early church (Alexandria, Antioquia, Rome, etc.) was fulfilled following this way.
At the same time, if I remember correctly, the greeks also interacted with a lot of people from the East. Therefore, these last two points can be applied here too.
As we know, the romans implemented the war to this complex equation, copying (to a great extent) the greek legacy.
Sorry if I made some grammatical mistake, but english isn't my first language. And sorry if this answer is incomplete, but I'm just an amateur on these matters. Hope it helps, though.
Ummm i'm no historian mate but l'm pretty sure that's not a thing. Maybe you need to narrow your question down to just architecture or empires?