Sorry if someone already explained, I'm on break at work and just scanning the boards
Let's say 50 people have fp and 50 are on standby. When they reach the attraction almost all 50 fp people will be let through while the standby continues to grow. There's stopping to ask how many, to tell people where to go and although this process seems like it takes seconds the constant doing of it builds time
Compound that with thr fact that the ride is operating at less capacity it means either they cut gp allowance significantly, meaning that they will be harder to get, or, they keep current numbers and thr standby will just balloon
With just standby operating now the process to load attractions goes much quicker and smoother and makes for a standby line that looks long but it's constantly moving.
This is even without counting the stoppage time due to cleaning