I don't have a source, but as a Dutch person: there is no legal way to actually grow cannabis in this country, nor to sell it to stores ("coffeeshops") that sell it to consumers. Thus all cannabis sold necessarily[0] comes from illegal growth operations, which is the kind of thing that's very lucrative for organized crime.

[0] if you want to be pedantic about it, owning up to 5 cannabis plants is decriminalized, but nobody is under the illusion that such personal use scale agriculture is fueling the commercial trade