FTS
Alexander did not create new empire, he just conquer it from Persia. What he wanted was to unify both culture, Hellenistic and Persian. He did not consider Persians inferior, but showed a deep admiration for the civilization. He also implemented policy of mutual interbreeding, where he forced members of his staff and armed forces to marry local women and created a blend of new class from both. This worked in Afghanistan. It was much harder for next generation to oppose Greek ideals when daddy was Greek. Romans did similar tactics on conquered, but not hostile tribes.
Mongol empire was build on opposite principles than Hellenistic empire. It was based on mass slaughter and population extermination. Mongols did not care about wealth, art, culture, business, and had very little need for urban population among nomads. Grandson of Genghis said that his grandfather could conquer the world, but he did not know a single day how to rule it. Mongol conquests were pretty much erased after 100 years. Because most of the genes were destroyed, the surviving population did not had much chance to restore to their previous greatness. One of the major regression of Orthodox Europe, Middle East, Central Asia, and Far East after 1250 in comparison to Western Europe can be traced to Mongol Empire. Western Europe became the wealthiest and technologically the most advanced region in the world for next 500 years.