I have also seen this 50% figure from various sources but I find it hard to believe, although I have no specific knowledge that would refute it. As far as I know, it's mostly low-grade weed that comes over the border, and while some grow operations within the US are cartel-run, I still am so surprised it would equal their profits from cocaine and methamphetamine combined.

I think another aspect is, that the mexican cartels can grow the marijuana themselves and gain full profit off it, while cocaine or coca plants/paste have to be imported from south america.

it is also vastly more popular