The 10th amendment says:

The powers not delegated to the United States by the Constitution, nor prohibited by it to the states, are reserved to the states respectively, or to the people.

That basically means: The Feds need a reason to pass a law, and that reason has to be listed in the US Constitution.

Ok... So what listed power lets them regulate home grown marijuana for personal use?

If your answer is "the power to regulate interstate commerce," then you missed the part about it being "home grown" for "personal use."

Even if you somehow find a way to link homegrown weed to interstate commerce...why did prohibition require a whole new amendment to the constitution to be legal if it falls under a virtually identical category as marijuana?

PLEASE DONT JUST SAY WHY MARIJUANA IS BAD!! i want someone to tell me how its constitutional to ban it.