Becasue you want the best. I have been in london for a week now and I have seen luxury cars every single day. The S-class is clearly the most popular luxury car. I'd say 70 percent of all large luxury cars I have seen have been the mighty s-class. The W140, W220 and the W221. Then 20 percent have been the 7er and 10 percent have been the A8, LS, Phantom and XJ. I have clearly been shown that the S-class is the leader.
The reason why the A8 wins some of the tests is becasue it is cheaper and gives slighty more value.