The problem with WWI is the same as WWII, Korea, Vietnam, etc.- they're all niche markets. WWI enthusiasts are admittedly a much smaller percentage of the market than any other period, for all the reasons Llama posted. Most of the non-Jet set gravitate to WWII, for the faster planes, better armament and more glamorous planes. Even though they usually fly them slowly and around and around in circles, like WWI combat was, not WWII.
I for one loved DoA for the same reasons Llama just badmouthed it- most of the people in there were ENTHUSIASTS, not GAMERS. You could get a good, long, hard fight in there and usually it would stay 1v1. A fight above any help usually meant that they were out of the equation, and a fight out of icon range meant any help was several minutes away. Plus with most planes being similar in performance, tactics meant much more that crank and bank fighting- anything you could do, your target could do. You had to outsmart him before you could outfly him. You didn't just scream in with your hair on fire and all guns blazing and expect to blow off your targets tail or wing with a half second burst- you had to WORK for it. And you had to learn to shoot, not just spray-and-pray. The difference between a guy with a 20% hit rate and guy with a 3% rate was obvious and dramatic. I often landed 4 or 5 kills with ammo left over once I learned how to shoot and aim in my ride of choice (Albatros D.V). The limiting factor was usually fuel and finding oppsition.
And I'm still a big fan of non-cannon armed planes. That is a BIG selling point for me when it comes to WWI.