The idea being that legal commerce would build up their domestic industries, improve their overall economy, and help drive a wedge between the people and the drug lords.
In practice though, and with "friends like Pelosi", it's probably a lot harder to do. But the theory has some sound foundations.
Money seeks the best return, or so we're told by economists. I think that's probably true. So the question is where is the best return on investment going to come from? "legal commerce" and "domestic industries" or cocaine.
Cocaine will provide the best ROI by far. It makes the farmers a helluva lot more money than corn or fake Nike's. Everyone in Columbia makes money from cocaine directly or indirectly. Cocaine drives the Columbian economy in a way that no other product or commodity will do.