atleast in ac,i think empty seats are due to large no of ac coaches.also i prefer this due to its timing in hwh. 4am early morning in hwh is awsome time to reach.
i think it has very less useful stoppage after bsp.i mean pple travel to rou or ckp from bsp,and as far as i know there is huge crowd for kgp that like to board at bsp.
overall due to high no of ac/sl coaches and low demand of onroute stations due to odd timing,its easy to get reservation.