McH, I have to admit, you make sense. I never really thought of culture as a determining factor in these kinds of things, but the more I think about it, examples pop up all over history--especially if you consider military doctrines to be a cultural trait, which does make sense in the case of the Romans and the Gauls, whose differing styles of warfare basically boil down to differing definitions of honor and glory. As I polish up on them, I realize that they DID have the resources to outfit heavy infantry if they had ever bothered to develop them, but they emphasized personal prowess over unit cohesion--no armor, no shield walls, no real cooperation.
Africa, however, is a different story. Here's a second question for you: Africa has hosted a couple dozen empires--a few of which are noteworthy. What do most of them have in common?
Now, to answer my first question... Rome was founded by racially-mixed Latins and Etruscan nobles who were excluded from the highest rungs of both societies. Rome's last leaders--and I say leaders instead of rulers for a reason--were Germanic nobles who were likewise excluded from holding legitimate power. In both cases, the old order had calcified to the point that they rejected men who were otherwise the best and brightest on the basis of race--so the greatest men of their generation decided they'd just start their own club.