Hi! Realise this is a bit late, but here's my take on the subject:

There's a lot of theory that cites economic, domestic, and international factors on this subject, and to my knowledge there's no single answer to this question, but in terms of structural problems, one of the main issues is addressed by what's known as Dependency Theory. Specifically, the branch that deals with commodity dependency. Dependency Theory is a pretty loaded topic among Latin Americanists, but it does offer an interesting perspective on this topic.

The premise of the theory–in relation to commodities–is that "developing" economies like those of Latin America are often kept down by structural inequalities in the global market. As countries rich in natural resources and cheap labor, but relatively poor in industry and technology, their natural place in the world's economy is to harvest and export those resources as cheaply as possible, importing finished products from more "developed" states. Without state intervention, countries never move beyond this stage because they never have comparative advantage in anything else.

The problem with this–beyond the implications for countries' independence/sovereignty–is that it perpetuates inequality. A small class (usually much whiter than the populace at large) owning most land and capital reaps the profits of these sales, while the vast majority of the populace works for incredibly low wages.

This institutionalised, structural inequality has resulted in the creation of stark class divides across much of the region, which in turn has lead to political polarisation and radicalisation. Sometimes this leads to misinformed policies on the part of leftists, provoking a militant backlash. Other times, reactionary coups spring out of unfounded fears of a Bolshevik-style revolution, aided and abetted by military opportunists. The hand of these trends can be seen in the 1970s coups of Chile, Argentina, etc.

A myriad of other factors can be cited–international intervention, colonial legacies, and the lack of strong institutions to name a few–but I think this is an important part of the puzzle.

Sources:

Coatsworth, John H. "Inequality, institutions and economic growth in Latin America." Journal of Latin American Studies 40, no. 03 (2008): 545-569.

Russett, Bruce M. "Inequality and instability: The relation of land tenure to politics." World Politics 16.03 (1964): 442-454.

Santos, Theotonio Dos. 1970. “The Structure of Dependence”. The American Economic Review 60 (2). American Economic Association: 231–36.