It might came from cosideration of actual operational issues: London to Baghdad is like 5:15 on 757/767 (in even more in case you going to BSR or KWI), basically 10:30 flight time plus you can expect 2-3 hours turnaround time at SDA, so it doesn't fit into 13 hrs, and in case of emergency you cannot manage to do a crew swap somewhere en-route, and sure you would not consider to have crews based in SDA.