Think about ALLL the laws that would need amending that come with recreational weed on a national level. Think about how NO other nation that at least I know of allows recreational weed on a national level.
Think of all the nations that allow medical weed now on a national level but not recreational still.
Let's say I got 12 of my friends to buy an ounce of weed. And we drove it separately or two people to a car across state lines to a state where we knew we could sell it quickly to make a big profit- how many ounces do we have total? 12? That's almost a pound. But we did nothing wrong- it's federally legal. All we did was drive it from one state to the next. If I can think of things like this- so will criminals.