Except for Costa Rica, it is my opinion that the failures of democracy in Latin America is due to an unfortunate history of Caudillismo. Before their independence, Spain dedicated itself to plunder its colonies as opposed to the American tradition of building something for themselves, which created the pillars of democracy: Individual liberties, a strong work ethic and a republican form of government.
After their independence, Latin American nations were and have been governed by strong men (Caudillos), who did to the people what the Spanish crown had done before - exploit them. It's the fear of liberty - better let others make the hard choices for you.
Marxism takes roots in the region because even if Cuba does not count with a Soviet sponsor anymore, its spreading of the marxist revolution is now financed by the drug trade.
One of the interesting themes of "Aztec Blood", by Gary Jennings, is that the initial governments of the new world installed by Spain were ones of institutional corruption -- the governor of an area obtained his position by paying a bribe to the King of Spain, and was in turn expected to make his fortune by soliciting bribes from those he ruled. The next tier of officials bribed him, and were in turn solicited bribes from the next level to them.
Corruption became an ingrained part of their culture, and has become an insurmountable impediment to freedom since.