They weren't, but like all big corporations, they diversified their product lines:

- Losing marijuana business, Mexican cartels push heroin and meth -https://www.washingtonpost.com/world/the_americas/losing-mar...

- Marijuana Legalization Sparks Heroin, Meth Trade Increase with Mexican Cartels - https://www.thefix.com/content/marijuana-legalization-sparks...

- Drug Cartels Turning to Meth, Heroin After Pot Legalization in U.S. - http://insider.foxnews.com/2015/05/19/drug-cartels-turning-m...

- Have Changing Marijuana Laws Led to an Increase in Heroin & Meth Smuggling? - http://lighthouserecoveryinstitute.com/have-changing-marijua...

Also, marijuana was never much money for the cartels, the real money has always been in heroin and meth. It's much cheaper to produce and the profit margins are much better than weed.

If you want to cut off their biggest sources of illegal activity, you're not only going to have to legalize some pretty serious drugs, you're also going to have to legalize kidnapping, murder, blackmail and racketeering. The idea that the only way cartels make money is on drugs is terribly myopic.

I'm not saying we should legalise all profitable things. But when the profit:social harm ratio is large, making them illegal may do more harm than good. Every extra product line helps the cartels - if it wasn't profitable they wouldn't be doing it. Kidnapping and racketeering are expensive and dangerous; drugs are not their only source of income but they are a big one.