One difference is that Greece was mostly a peninsula, with many cultural centers in nearby islands. So the Greeks were much more capable sailors than the early Persians. Also, Persia was close to Mesopotamia, at that time one of the most advanced areas in the world, and relatively highly populated since urbanism there started so early. The threat of invasion from Mesopotamia in the west as well as nomads from the steppes probably influenced the rise of strong central government in Persia.
I hope that helped a little.
