The correct answer to this open question is the following.
They wanted to explore new territories to get as many natural resources and raw material to increase the wealthiness of the Spanish crown.
Cabeza de Vaca was the explorer that opened the door for further explorations of Hernando de Soto and Vazques de Coronado. Hernando de Soto got to Mexico in 1539 did the same in Cuba. Cabeza de Vaca was the first explorer to describe the coastal areas of the Gulf of Mexico, and then explored New Mexico, Texas, and Arizona.