You severely underestimate the kind of power and infrastructure these people have.
It's a separate problem with no solution unless profit dries up.
Edit: Looks like pot is actually the #1 crop in the US http://www.drugscience.org/Archive/bcr2/cashcrops.html (if their numbers are accurate.)
Pot cultivation went way up after 9/11 due to the increase in border security. It just isnt cost effective to smuggle a kilo of pot versus a kilo of coke. Pot is pretty difficult to move in large amounts so it make sense to grow it north of the border. Wholesale pot might go for 2-3k per key while coke might go for 10-20k per key.