The recency (I checked it, apparently it's a word) of mass political participation in political life, the military element of liberation from colonial rule, the impact of the 1930s world Depression and later Cold War geopolitics.

The region had been under Iberian imperial rule until the revolutions of the 1810s-20s, during which time most of its population was inevitably excluded from government. At least in formerly Spanish-ruled lands, the revolutions left a legacy of caudillismo - strong leadership associated with nation-building and supported by networks of patronage. This was giving way in the early 20th century to a shift to electoral politics (still often infused with an element of charismatic leadership), but the crisis of 1930 intervened, bringing down the commercial basis of regional prosperity and confidence in the emerging liberal order. Subsequent US fear of communist influence helped to maintain military regimes in place into the 1980s.