But not all, thus violating the sovereignty of the state that doesn't want to prohibit it. If California wants to be the state that attracts all the stoners, then so be it. That's within the power of the people of California.
If California or another state legalizes marijuana or slavery or the manufacture and private possession of weaponized anthrax spores, hand grenades, or truck bombs, it subverts the respective federal and state bans. No system could survive in which one state could permit and enable what the federal government and other states categorically prohibit. Yet that is what your view of the Commerce Clause supposes.