* ACIII is does not provide same comfort as ACII and is preferred solely because it costs lesser. ACIII is more similar to SL than ACII in matter of space. Relief from heat is the common factor between the two.
* It is the ac facility which is essential for long and short time day journeys during hot/very-hot months (Mar-Oct).
* Poor maintenance/ cleanliness/ hygiene of non ac sleeper coaches and dust ingress during the journey further promotes patronage of ACIII
*...
more... The main reason for lower popularity of ACII is higher price making it out of reach of majority of people. Those who can afford ACII avoid ACIII just because it is not comfortable.