You mean why the colonies grew apart unlike Portuguese America(Brazil) and English America(USA)?

Benedict Anderson addresses this in Imagined Communities, his (excellent) book on nationalism—it's actually an especially important question for him, since the formation of nation-states in Latin America actually precedes that of their European counterparts to some extent, and is thus vital in understanding the process.

A root cause might be geographical immensity, and the ensuing division of Spanish territory in the Americas into separate administrative units from the 16th century onward. Crucially, because all trade had to go through Spanish ports, these units were economically isolated from one another as well.

The main political factor that Anderson sees as crucial for the transition from an economic zone to a region with its own cultural identity, though, is the fact that— even in the ostensibly meritocratic system of advancement that constituted the empire's bureaucratic administration—creoles (people of "pure" Spanish ancestry but born in the colonies) were almost never chosen to make the crucial journey across the Atlantic to join the ranks of the highest elite. Instead, they invariably found themselves in the capital of their respective administrative unit, unable to move further up the ladder but now, significantly, in the company of a number of like-minded young men who—like the new colonial upper class as a while—are charged with carrying out the Empire's administrative power even as they're being kept in check economically and politically.

This might have remained limited to a creole upper class, but the advent of newspapers in the 18th century—widespread geographically but limited by the aforementioned distances and technology of the time from overcoming their regional locations—then played a crucial role in cementing these identities across a larger section of the population (the main thesis of Anderson's book is actually how mass media creates this kind of shared identity).

Easy: All were not part of the same colony.

Almost since the beginning, and for a more efficient administration, the Spanish empire in the Americas was divided in two viceroyalties, each with a different viceroy who was directly appointed by the Crown and governed in its stead: New Spain (Mexico) and Peru. In the 18th century two more viceroyalties were created out of peripheric territory of the Viceroyalty of Peru in South America: New Granada (modern Ecuador, Colombia and Venezuela) and La Plata (modern Bolivia, Paraguay, Argentina and Uruguay). Each viceroyalty, in turn, had further divisions like general captaincies, audiencias, "kingdoms", etc.

So, when the independence process begins in the early 19th century, people in one viceroyalty are effectively removing the viceroy out of power and cutting ties with the crown. The people in Mexico, however, have no reason to follow the people in Peru, or Argentina, etc, since they had never followed orders from there in the first place.