Because they are in fact driven by profit. They still can not afford to fuck with the american law enforcement or military. Look at what happend in 84-85 DEA agent was kidnapped from right in front of the consulate, tortured for thirty hours and then buried. The reataliation from the americans was enough to teach them a lesson they have learned well, for the most part. Remember there are less than 10, i think far less maybe 5 or 6 serious cartels running shit down there. With thousands of various gangs. LEO can stategically hit those cartels in retaliation and leave the others alone. In fact thats what the mexican government seems to do quite often, and they favor those cartels cause of their own obvious connections and relationships.
) should come up with a system for "prescribing" certain amounts depending on condition and severity of condition. Because we can't realistically expect someone using cannabis for insomnia to be growing 150 plants and holding 12 lbs for himself. I also don't think that plant count is the best way to do it, I think canopy size is a better judgment of potential yield (think SOG vs growing trees.. either way you'll get about 400 gm/m2). But I'm beginning to ramble and get off topic so... I digress 
