Drugs might be easier to arrest people for. That doesn't justify the war on drugs, but it does raise the possibility of having more people who should be in jail not being in jail.

Zealous prosecution of drug crimes also raises the possibility of having more people in jail who aren't involved in any other criminal activity.

I would wager most (or at least a very large %) people involved in drug crime on the supply end, or on the demand end of harder drugs commit other offences or are likely to in future.

But most of the people in jail aren't on the supply end. That's the point.