They can try, but it should not be possible for a criminal cartel to corner the market on a legal product. They'd need to hunt down and kill or intimidate every single person who could grow a plant in their backyard, or else their inflated prices would be undercut. The amount of crime they'd have to do to corner the market would be far more heinous than with the illegal market. I don't think it's a war they can win. Plus, the whole reason the cartels were successful is that most Mexicans supported them, because Mexicans saw them as pro-Mexico and anti-US. But if the cartels are seen as primarily preying on and harming Mexicans, the people will turn on them, and they'll be hunted down.