Here is my take on it, there simply is not enough to do to keep people interested as it is now.
That is not to say that it needs to be a a field capture, win the war setup.
But there needs to be an alternative to the furball.
With the current low numbers, unless your one of the top 4 sticks in the arena at any given point eventually you will get frustrated. Would you really fly in LWOrange if there were only 10 pilots in there, and 8 of them shot you down every time you saw one? Ohhh for a tour you might, but after 3 tours? Every time you did manage to get a kill one of the pro's swoops in and takes you down. How long would you keep coming back?
Rather than leave the arena, wouldn't it be better if there was "something" else to do?
Be it zep hunting, balloon busting, convoy strafing, etc.
That doesn't mean it has to be tied field capture, or to winning the war, resetting the map, etc.
It just means there needs to be alternatives. Something to do other than furball. So when the next 3 guys look at the arena select screen, instead of seeing 8 guys, maybe they see 14 or 20. And the next bunch see 25 or 30. People will draw people, if you can keep them interested, keep them busy flying and having at least some fun.
Also when there are lots of people in there, the odds are much better that you can get a kill or 2 and get out to land them. You still have the same 4 top sticks, but there are also a lot of average pilots and probably some new guys. So the percentage chance of winning a fight is much higher. When its under 10, those odds go down dramatically for the average pilot.
Its a bit like hitting your head against a brick wall.
You try it, and it hurts, but you really want to do this and get through, so you try it again.
And you keep trying it until the pain level eventually gets high enough to override your desire.
Its the same with the WWI arena. Eventually either you master it, and become one of the 5 or 6 top sticks, or the pain and frustration drives you away. Personally I loved the first tour, 2nd tour started hurting, and by the end of the 3rd tour I'd look at the arena screen and alt F4. I would go do anything that was less frustrating and painful.
More things to do, that involve flying, that take a certain amount of skill, and daring, with a certain amount of risk, and that leaves you feeling ready to maybe go take another crack at that furball.