Decolonized Latin American countries were invariably established as democratic republics (particularly if you consider the monarchies of Brazil and Mexico to be part of their de-colonial transition rather than post-colonial states).

However, as you observe, Latin American countries have been more susceptible than the US to detours into authoritarianism.

Probably the most important underlying reason for this is the very large wealth gap and its racial correlation: relatively small white populations holding most of the assets and technical expertise of a society numerically dominated by mixed, native and African-descended people. This situation created a big incentive for the elite to sideline popular democracy or (one could argue with respect institutions such as the PRI in Mexico) subvert it with nominally populist slogans and platforms. In the US, by contrast, the inequality wasn't as severe, and the elite were demographically identical (white Christians) to the majority of voters.

Since World War II, this inequality also made Latin America ripe for socialist revolution, which created an environment for authoritarian rule both by socialists (Cuba, Nicaragua, Venezuela) and their enemies (the juntas and Pinochets of the world). Socialist revolution was never much of a threat in the US, although of course part of why that was a 40 year period (1933 to 1973) during which a mix of deliberate public policy and luck dramatically decreased inequality in the US; developments that did not have much in the way of analogous changes in Latin America.