Darkness only gets worse, doesn’t improve or possibly blow over, break up. You know the condition is coming for sure, just don’t know when. Rain is tricky as it can be green right over you but not making it to the ground. OR not even show up on radar and be raining. Teams have radar and have as much info as NASCAR And probably monitor the corner workers radios as well in those situation.
And it takes time for the corner observer to make decision it’s too wet, to call control, for control to put out the caution, and drivers to recognize and slow down. It‘s not an instant process. Bad break for leaders but it happens.