See, I don't know ... DPR was definitely a big fish, but only in a certain context. He obviously had the technical chops (and moral compass) to set things up the way he did, but at the end of the day, all he did was facilitate business for other (IMO) much bigger fish.
I'm not in any way justifying his actions or anything like that. But I too was surprised that they just shut it down instead of just assuming control and going after what could have turned out to be cartels (assuming it wasn't just small time growers, etc.).
More likely than not, some of the drugs sold on SR were produced by cartels. The SR sellers were probably so far down the supply chain that they could not have given the police any useful leads on the cartels.
In case people have forgotten, SR itself was a target. There was pressure from Congress to shut it down, and it was probably the only goal of this operation:
I'd imagine there were a relatively small number of well-rated suppliers servicing the majority of trade, and fewer still producers. If DPR was in fact taking a ~10% cut, those suppliers would have stood to make considerable sums of money given the profit margins involved.