Why was tenochtitlan built in the middle of lake texcoco?

Tenochtitlan was build most likely built on a lake for protection. Because it had very narrow access bridges, it prevented the city from being attacked in mass because large groups of soldiers could not get in a single time. Not only that, but the mud from the lake bed was used as a fertilizer in their floating gardens, called Chinampas, which meant that the Aztecs could supply their own food even under a time a siege. Because the crops were also located on the water, it made it hard for enemies to damage them.