One reason could possibly be due to 77L flying through Saudi airspace vs 351 over Iran.

Flight data for past few days for both types of aircraft.

B350-1000 Qatar Airways flight QR729 - Flightradar24
6/10: 14:40
6/9: 14:37
6/8: 14:49
6/7: 14:49
6/6: 14:41
6/5: 14:32
6/4: 14:27

B77L Qatar Airways flight QR731 - Flightradar24
6/9: 15:47
6/8: 16:05
6/5: 15:59