The Persians did not think Greece was important because Persia was so much larger and prosperous. Persia was expansive and Darius was considered a king of kings. Persia's complex political hierarchy and expansive trade routes made Greece look like a bunch of poor rebellious island dwellers. The only reason that Persian fought Greece was to set an example on how subordination would not be tolerated. Even though that failed. Bulliet makes points about how greco-persian culture was similar in ways but I think that the political differences made it so that they could never meld.