How did former Spanish colonies in the Americas become so fragmented?

The truth is that more Spaniards colonized america after their independence than before.

Between XV-XVIII Spain colonized those lands with 750.000 Spaniards in overpopulated lands. Majority of them men were soldiers, marines and low-class class Spaniards and of course the nobility that occupied the elite. The majority of the population were "mestizos" the mixture of the race between Spaniards and natives.

However, after their independence, between 1857-1930 was the period of the great Spanish colonization.

In details, 4.600.000 settlers. 600.000: Puerto Rico and Cuba. 2.000.000: Argentina. 750.000: Brasil rest in small groups to other american countries.

30s, after the civil war 1.000.000 Spaniards exiled: Francia 500.000.

Argentina, Venezuela and mexico arrived the majority of the rest.

So at any case, the reality is that the Americas more populated country by Spanish settlers is Argentina.

This question has already been answered, but no-one has brought up the issue of compactness. The US, Canada, and Brazil have vastly more compact shapes and therefore many landlocked subregions and multiple internal routes of travel. By comparison, Spanish America is incredibly elongated. Almost all subregions are on the coast in the line described by the Panamerican Highway.

At the time of Latin American independence, only a few land routes connected the colonies. Long-distance travel was subject to native raids, and the Spanish's cherished horses couldn't cross areas without pasturage, making the Sonoran and Atacama deserts practically impassable.

In California, the first contact with Lima merchants and Rioplatense raiders happened only in the last decade before independence. Veracruz and Havana were well-linked by packet-boat throughout the colonial period -- but only in that direction, because the return route was to Spain and back.

If you study Brazilian History as well (I am Brazilian and I have read some very good Brazilian history books), you see that in Brazil many of the provinces had separatists feelings, in several occasions along the time. I will not mention examples, but there are dozens of rebellions that happened along the XIX century. And even in 1930 we had an armed "Revolution" in São Paulo state, with strong "autonomous" feelings.

But the fact that we had a "strong" personality king in the XIX century (arguably not a wise king, but a vigorous one), especially on the early days of Independence, that repressed the rebels, helped to keep the union. In many other occasions, the "central government", based on Rio de Janeiro, managed to contain other revolutionary instincts/actions as well.

In resume, when one reads about Brazilian history, one can realize that for several occasions we were on the verge of getting fragmented in a similar way to what happened with the Spanish colonies, but for many particular reasons, along the time, the central government managed to keep the unity.

One reason is because of the poor topography, and the lack of good transportation. Take the southern cone, for instance. The Andes Mountains divide Argentina and Chile. They also divide Colombia and Venezuela further north.

One kind of wonders why Uruguay and Paraguay are separate entities from Argentina, until one realizes that they formed around Montevideo and Asuncion respectively, and are badly connected to population centers in Argentina and Brazil. (Plus some 18th century Jesuit priests trained "local" Indian armies to repel invasions from invaders from the other two countries.)

Peru and Bolivia might have logically united, except for opposition from Chile.

Unlike Brazil, which is relatively compact, the Spanish speaking parts of Latin American are basically strung out in a long, thin, line. Even the six or seven central American countries basically consist of settlements along the coasts, with jungle in between DISconnecting the main cities (and hence countries) from one another.