It's been a few years since I've had any class on this topic, but IIRC the Washington Consensus didn't damage Colombian economy or society nearly as much as it did in the rest of the Andes, so the population wasn't nearly as motivated to support the "populist-leftism" that became widespread throughout the region. In addition, in recent years concerns about the FARC and Chavez's militarism have also prevented people like Mockus from being elected.
According to a (ten year old) article by Charles Bergquist from UoWa some reasons are (were):
High relatively stable 20th century growth rate fueled by coffee boom, which was mainly controlled by ordinary coffee farmers not latifundos, which created a large rural middle class.
Avoided most of the debt crisis in the 80s - neo-lib policies came later and was less hard on the poor than in most other Latin American countries.
He says less populism both left and right has been weak as a result of this. Liberalistic economic consensus instead.
But he also states that Colombia's income distribution is among the most unequal in South America only matched by Brazil. So the potential for the Left should be there.
Seems to think the Left uses violence because they are marginalized - which makes sense.
Still the difference to the rest of the continent seems to be getting smaller in the last decade and with the break-up of the Liberals maybe we will see a "normalization". The unequal income distribution should give some potential for this.
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