This has alot to do with the organizational structure of both Portuguese and Spanish colonialism. Except for the very earliest period, Brazil represented a coherant colony. It had subdivisions, but administratively it was a single entity. In Spanish America from at least the 1530s, the colonies were divided into a variety of different viceroyalties which were as distinct administratively from each other as they all were from Spain's kingdoms.

If you trace Spanish American independence the initial divisions (and even later divisions occurring in the decades after independence) maps along those earlier administrative divisions. For example, Mexico was the Viceroyalty of New Spain. When that entity gained independence and became Mexico it retained its previous administrative boundaries, the only exception being Guatemala and Central America which splintered off and formed the Republic of Central America (which would later splinter). This split made sense because Guatemala was an administrative center of its own (secondary to that of Mexico City).

Similarly, the viceroyalty of New Grenada (Panama, Colombia, Ecuador, Venezuela) became Gran Colombia - later to split into the present countries. The viceroyalty of Rio de la Plata became the United Provinces of Rio de la Plata (Argentina, Paraguay, Uruguay) later to split. But for the most part the modern nations of latin America represent the major subdivisions of the Viceroyalties of the colonial period.

In contrast, administration in Brazil was more centralized and therefore less prone to subdividing overtime. It also helped that Brazil gained independence as part of a dynastic split and not a true independence war.

A good textbook resource for these issues is Lockhart and Schwartz's Early Latin America.

What do you mean? Do you mean: "Why was Portugal able to hold on to it's colonies?" or "Why did Spain's colonies break apart into different countries after receiving independence?"