Why was Mexico City built where it is?

Answer 1

Spanish conquistadors built Mexico City atop the ruins of the former Aztec Empire's capital, Tenochtitlan. At the time, Spaniards found the area to be extremely fertile, and had actual roads connecting most of Central Mexico to the city. They saw the opportunity to cut costs by building the new capital of the nascent colony of New Spain at this site.

Answer 2

Because it was the capital of the Aztec civilization, and when Spanish conquistadores conquered the Aztecs in the early 16th century, they choose to found the capital of the New Spain in the same place.

As it has always been the most important economic, political and cultural center in the country, when Mexico won its war of independence the revolutionary government also choose to found the capital of the new nation in Mexico City.