So why the disparity between Athens and Rome?
There are both environmental and cultural reasons for this. Rome and the Italian peninsula had similar experiences to Europe in the middle ages with respect to development of their political systems. This similarity is based on two factors: personal leadership and a collective unity and equality of tribesmen. In other words, their political systems grew out of leadership based on personal charisma which encompassed regal, military, and political elements. The society was flat with a leader and his associates on top and everyone as equals below.
The early Greek experience was different because it was influenced by unique factors: tribal kings were weak financially, the Aegean was isolated geographically, and Greek life was simpler than Roman life.