The major reasoning of Spain was that they hoped to get rich: especially they hoped there would be Gold in the new World.
After this did not succeed ( they did not find as much Gold) they tried to use the land for economic reasons in other ways, by having plantations and the individual conquistadors were happy to possess a reasonably big pieces of land on their own. Also, simply by increasing their territory they gained on wealth and importance.
Also, they hoped to gain favour with the Pope over converting the indigenous people to Catholicism.
