Not only because they were NOT full blooded Spanish but because they were NOT born in Castille. Spanish Rule had its own rules, discriminating against people not only on who their parents were but where they were born. This is why it was the Jesuits and Franciscans that kept the Spanish empire together. Within a generation of these two groups being banned you saw the the independence of most of Spanish America. The Secular Government imposed by Spain was NEVER in complete control of Spanish America. It was the Jesuits and Franciscans that reported to Spain what was going on in the Spanish Colonies.
This started with the"Black Legend" i.e. the report on HOW bad the Spanish were treating the Natives. The Black Legend starts with a report on mis-treatment of the Natives by the Dominican friar Bartolome de las Casas, the religious (by this term I mean the Monks, Friars, nuns and Priests of the Catholic Church) tried to stop the excesses of the Spanish. Do to this good work the Religious had a good reputation in the New World until the Spanish Crown decided to secularized their lands in the mid 1700s. This was more a means to steal land for the king's favorites but it had the effect of striping the religious class of the money and ability to communicate with the Spanish Crown about what was going on it their part of the Spanish empire. Within generation New Spain had Become the Republic of Mexico, and within Two Generation no Spanish troops controlled any part of Continental North or South America (Cuba and Puerto Rico will stay Spanish till 1898 but those were islands in the New World).
Just some background on the General discrimination and Communication System of Colonial Spain in the New World.
Some background on the "Black Legend":
http://en.wikipedia.org/wiki/Black_Legend