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

They are most likely diversifying (or have done so by now). Which means every year the USG allows them to maintain their monopoly on pot and other drugs just makes the cartels stronger.

Volume vs. profit margin. Somewhere around 20 million people in the US use marijuana, which is a pretty damned big market.

I was watching a documentary and I believe I saw that 60% of Cartel income is from marijauna. Sorry I don't have a source.