Keith Humphreys claims that pot isn't particularly profitable for drug cartels:
Their big money comes from cocaine. They also make a lot of money from other things. [Legalization in Colorado and Washington] is a pinprick in terms of the Mexican cartels
He later expands on this argument:
Marijuana gets outsized attention in US drug policy debates, yet it matters at most slightly for the security of Mexico (and not at all for Central and South America). Domestically, it does not contribute to overdose deaths nor account for even 1% of imprisonments. But its status as a culture war symbol — particularly for baby boomers — will keep it in the forefront of popular debate even as concern over cocaine, methamphetamine and heroin wanes.
The Dish 