It's called the nondelegation doctrine, which forbids one branch of government from authorizing another branch of government to exercise its functions.
Because Article One vests "all legislative powers" to Congress, they cannot delegate legislative powers to the Executive and Judicial branches (because then not all legislative powers would be vested with Congress).
Because Article One vests "all legislative powers" to Congress, they cannot delegate legislative powers to the Executive and Judicial branches (because then not all legislative powers would be vested with Congress).