Probably because if it arrived back at DOH late afternoon, most of the passengers would be hanging around for the next big bank of westbound departures at c.midnight-2am. There may be costs to that in terms of additional lounge consumption and STPC. The current timings allow short connections onto the morning bank of departures. A late afternoon departure ex-PKX (and much longer connections in DOH) may also be a harder sell locally than a night time departure, potentially requiring lower fares. No doubt they have done the revenue numbers and the current timings work best. Some aircraft also sit on the ground for extended periods in places like BKK.