The Spaniards came to North America as conquerors--conquistadores--and to spread the Catholic religion, amass fortunes of wealth in gold and silver, and increase the glory of Imperial Spain.
Contact with native tribes also meant the introduction of disease, and often a 90% mortality rate for the affected tribes. This made Spanish rule a much simpler proposition. Another important factor was Spanish technological superiority in weapons, as was the introduction of their bloodline to form a new race, the Mestizos, that fostered increased subjugation and assimilation among the native peoples.
Their system of rule was organized, strict and widespread, encompassing the majority of South Central and North America at one time, with remnants of Spanish culture in terms of language, religion and traditions still dominant and visible today throughout Latin America.