Why do you say that? Anecdotally there are tons of cannabis listings, and from a quick search I found this study [0] (Fig. 3) showing cannabis was easily a plurality of drug sales on one darknet market (31%, with the next most common being dissociatives+psychadelics, which is also a broader category, at 22%).

[0] https://www.emcdda.europa.eu/system/files/publications/13042...