There is just way, way, way too much money to be made. One example is that even in deeply rural inland areas of the US, more meth is from Mexico than local areas.
The only major drug rackets they don't have controlled are ecstasy from Israel than Holland, and Heroin east of the Mississippi which comes out of the "stans" in Asia and moves on to Turkey and then different groups in Europe or Nigerians.
Interesting link on "The Balkan Route" of Heroin into Europe.
http://serbianna.com/analysis/archives/1311