Because, people can easily grow it and if caught with it, so what? Its legal. No fines no nothing. Second, when growing it the goverment loses money due to the fact its not being payed for, which means it doesnt have taxes. Sure, they could legalize the possesion of it, then people would just sell it and still.. the government makes no money, they could keep the growing of it illegal.. so what, its illegal now and its still out there which means people still grow it.