After Tenochtitlán became independent from Azcapotzalco (thanks to its poticial instability and Tenochtitlán's alliance with Texcoco and Tlacopan), its territory grew considerably, becoming the center of a new empire. One of the main reasons why it became one of the richest cities in the world was the tribute that flowed in from other subordinant cities, which made the inhabitants wealthy. Also, distribution of land eventually caused a rapid immigration into the city.
