The reality of the fact is that if you made it legal so sell, own, or buy. You would still need to make it illegal to grow. Otherwise there wouldn't be much point to any of it. Then you would still need a policing agency, regulations and so on.
Any money that could potentially be made would have to be reinvested in trying to ensure that the US government brand of marijuana is the only kind you could buy.
Basically it's a double edged sword. Keep it illegal, keep spending millions a year policing it.
Legalize it, still have to spend millions a year trying to protect your investment in it.