It's called "enabling legislation" or "enabling regulations".
In other words, all agencies have to be told exactly HOW the sanctions are going to be implemented and/or enforced.
If you pass legislation imposing banking sanctions on Russia ... what, exactly, does that mean to businesses?
EG: Does it only affect banks giving money TO Russia ... or does it also mean individuals can't have accounts in Russian banks?
Since the White House is the Executive Branch, all agencies have to get that information from the White House before they can put any regulations in place. Which means somebody at the White House has to either interpret the law themselves or meet with Congress to determine what the intent was. And then they have to get the President to issue the orders to all federal agencies.