Answer:
Hernando de Soto covered so much land because he wanted to find a city like Tenochtitlan or Cuzco.
Step-by-step explanation:
De Soto believed that to the North of the American continent, he would find another large city full of wealth like gold and silver like those that were found in Mexico and Peru. This is why he took with him a relatively large number of explorers (around 700 men, equipped with weapons and horses).
De Soto arrived in Florida, but quickly found out that the Peninsula was devoid of gold, and full of mosquitoes and swamps. He did not have better luck to the North, where he was almost killed by Appalachian Indians.
The last stages of his trip were it what is now Arkansas and Texas, but he was unlucky because he never found the large wealthy city he was looking for.