Why did the Europeans introduce racism in Mexico?

During the colonial period,the Spaniards imposed the idea that social status depended not on one's level of education or wealth but on one's race.The Whites were "superior" to all,followed by Mestizos(of mixed White & American Indian ancestry),further down the social ladder were the Mulattos(of mixed White & Black ancestry),Zambos(of mixed Black & American Indian ancestry),The pure-blooded American Indians & Blacks were considered by far the most inferior.