All classes of this train command a good patronage, so need for longer train. 3E is less preferred as compared to ACIII coaches. Earlier I have observed many times that W/L of acIII started earlier than 3E and after that 3E seats got fully sold.
Journey in this train is strictly for sleeping, sleep at midnight and wake up at destination. It is the comfortable sleep that matters most. Early comers (first 60-65%) can get long berths in 3E (which are same as acIII berths) at lesser price than acIII, for them 3E may be preferable. But late comers are sufferers who are compelled to sleep on the cumbersome & congested side berths.