An overlooked factor is that the American Revolution was largely a “white” movement, due differences in how the British and Spanish settled lands.
The Spanish were partially motivated by evangelizing for the Catholic Faith, thus they were more willing to integrate the native populations. The British, however, were more interested in economic development or escaping persecution, and thus more insular.
Taking Connecticut as an example, Colonial Connecticut originated as three separate settlements; Hartford, New Haven, and Saybrook. The colonies traded with the Indians, but did not attempt to govern them, broadly speaking.
In Spanish territories, missionaries were often sent in first, followed by traders and administrators. While Spanish rule was by no means gentle, the Indians were specifically incorporated into the society, and intermarriage and other integration occurred. When massacres did occur, such as against the Aztecs, the Spanish were motivated at least in part to defend the various tribes oppressed by the Aztecs (from which the Aztecs held ritualistic wars to capture thousands of prisoners for human sacrifice).
The allied tribes often maintained a large degree of autonomy under Spanish administration. Thus, when independence occurred, the various Spanish colonies reflected nations that predated the colonial times by hundreds or thousands of years. Each nation had distinct cultures and historical baggage.
The United States, by contrast, had a largely homogenous White population which chose to split from an oversees White administration. The Indian Tribes were split on the issue, because they were separate parallel societies looking out for their own interests. Some allied with Colonists, others with the King, others fought both sides, and others stayed neutral.
The United States had a fairly small population, and the British Empire was at the height of its strength. Unifying helped pool their resources to fend off the British and other potential plunderers. The American Revolution had a homogenous population with shallow roots in the area, making federal integration an obvious solution.
The South American nations, however, reflected ancient tribes and national identities, which did not beg for integration. They became independent as teh Spanish Empire was waning, and the United States was growing in power. Under the Monroe Doctrine, the United States considered itself to be the protector of the Americas, and continued European intervention was unwelcome. Rather than unite to fend of hostile outsiders, they had theoretically benevolent outsiders to keep them relatively protected. These nations became self governing under vastly different circumstances than the United States, and forming a federation or confederation was not as compelling a solution.