After reading all of this and more it's my opinion that
A) Colombian, Peruvian and Bolivian cartels all have laundered their money to buy massive cattle ranches or large armies to control the land
B) Because of this and obviously they have access to other restricted chemicals, they are able to purchase ton quantities of levamisole
C) Someone (a chemist, a vet) realized that levamisole is a nicotinic agonist
D) Profit => cigarettes in my cocaine! Gimme another!
Plus I'm sure whoever did suggest it as a cut to the cartels also pointed out its antimicrobial properties. Or maybe that was the main reason, and the nicotinic activity was the added bonus. Either way there's at least 2 very good reasons why it's being added despite its price.