Cartels make far more money off "hard drugs" than they do off marijuana, and smuggling hard drugs is easier just due to size (a kilo of cocaine is tiny compared to a kilo of marijuana) and relative lack of tell-tale signs like odors (dogs can sniff out packaged cocaine and meth, humans can sniff out packaged marijuana). I served on a grand jury a couple of years ago and a DEA agent, testifying in a big meth smuggling indictment, said the local authorities had cracked down so efficiently on small, local meth labs that it opened a big door for cartels to become the suppliers. Violence and volume rose rather immediately. I think legalizing marijuana is probably a good step, and it will take >0 dollars from cartels, but I highly doubt it would be a serious impediment to them, the will continue with other drugs, local (to Mexico) activities like kidnapping and extortion, and branching out into other organized crime activities (fraud, loansharking, etc...)