The reasons for Latin America's turn to the left are not hard to discern. Along with many other commentators and public intellectuals, I started detecting those reasons nearly fifteen years ago, and I recorded them in my book Utopia Unarmed: The Latin American Left After the Cold War, which made several points. The first was that the fall of the Soviet Union would help the Latin American left by removing its geopolitical stigma. Washington would no longer be able to accuse any left-of-center regime in the region of being a "Soviet beachhead" (as it had every such government since it fomented the overthrow of Jacobo Arbenz's administration in Guatemala in 1954); left-wing governments would no longer have to choose between the United States and the Soviet Union, because the latter had simply disappeared.
The second point was that regardless of the success or failure of economic reforms in the 1990s and the discrediting of traditional Latin American economic policies, Latin America's extreme inequality (Latin America is the world's most unequal region), poverty, and concentration of wealth, income, power, and opportunity meant that it would have to be governed from the left of center. The combination of inequality and democracy tends to cause a movement to the left everywhere. This was true in western Europe from the end of the nineteenth century until after World War II; it is true today in Latin America. The impoverished masses vote for the type of policies that, they hope, will make them less poor.
Third, the advent of widespread democratization and the consolidation of democratic elections as the only road to power would, sooner or later, lead to victories for the left -- precisely because of the social, demographic, and ethnic configuration of the region. In other words, even without the other proximate causes, Latin America would almost certainly have tilted left.This forecast became all the more certain once it became evident that the economic, social, and political reforms implemented in Latin America starting in the mid-1980s had not delivered on their promises....
Compare this article from the NY Times on the erosion of the old South American political parties.
From Venezuela to Argentina, many of the traditional parties that built dynasties through patronage and hard- knuckle politics - but also offered stability, a clear ideology and experienced functionaries ready to govern - are disintegrating. Disillusioned by corruption and a failure to deliver prosperity, voters are increasingly captivated by new, mostly leftist movements that promise to redistribute wealth, punishing traditional parties and turning political systems on their heads.