I would assume from this you've not used this service? I've had to stand from Nottingham to Norwich on more than one occasion because it's been full. When we used to have 3-car 170s, it would often be full too.
The main reason for the lower passenger figures is the ticket prices suppressing demand. I often travel via London than go direct between Sheffield and Norwich now because it's cheaper. Central Trains admitted in the mid 00's to the press that due to a lack of suitable stock they were having to raise prices to reduce demand for seats.
If the stock existed, I'm confident that EMT would run 4 cars throughout the journey.