gold.
Gold was known to exist in the Americas.
There they thought they could find a ton of gold. They did, and they also found Pre-Columbian cultures to conquer and convert.
Most Spanish explorers were drawn to the Americas in search of wealth, particularly gold and silver, as well as new trade routes. The promise of vast resources and the potential for land acquisition motivated expeditions. Additionally, the spirit of adventure and the desire to spread Christianity among indigenous populations also played significant roles in their exploration efforts. These factors combined created a compelling incentive for Spanish exploration in the New World.
Spain
Spanish explorers such as Hernan Cortes and Francisco Pizarro claimed the most territory in the Americas for Spain. Cortes conquered the Aztec Empire in present-day Mexico, while Pizarro conquered the Inca Empire in present-day Peru.
Smallpox, and other unfamiliar illnesses. Disease was spread by most if not all of the explorers to the New World.
Most likely, since most of the explorers in that area were Spanish, it would be Catholicism.
Portugal was limited by the Treaty of Tordesillas (1494) which gave Spain the right to colonize most of the Americas. The exception was Brazil, the easternmost part of South America. Portugal, however, also conducted trade, conquest, and piracy in India by sailing around Africa.
The Spanish established colonies in the Americas long before many other European countries because they were the early pioneers (or, pathfinders) in the Age of Exploration. With the inclination and income pertinent for supporting explorers, they sent out teams ready to colonize (and conquer) well before most of their European rivals.
Disease
Disease
Most Spanish explorers practiced Catholicism and were motivated by a desire to spread Christianity to the New World. They also engaged in trade, seeking new trade routes and resources for Spain. Many explorers were driven by a thirst for glory, fame, and wealth.