No, it's not the money. There are enough crimes to keep the police and prison machine running.

Drugs, however, are unique because everybody consumes them. Since everybody consumes them, you can always find something in any search you conduct and the charges will stick. So they are excellent tools to put any political dissenter or rival behind bars; and to implement random search policies in all sorts of places. Both are about power, not money. Many other crimes could generate as much or more money. None generates so much power.