Despite claims to the contrary, in all of the other major games systems (Warhammner, SFB, Battletech) DYO is a big part. The only real question about ASL is why are so many players wedded to scenarios as opposed to DYO? All of the other systems have scenarios, yet DYO play dominates. Warhammer, which by the way has a much greater following than ASL has ever had by perphaps a much as 10:1, is almost exclusively DYO.
The short answer is: With all due respect, this is like comparing apples and carrots.
Those of us ASLers who are wedded to scenarios instead of DYO play the game because history fires our imagination, and it does a particularly good job of bringing that history to life. See also the current threads on "Fictional Scenarios", "Piepsk"...
ASL is a game about a real history, with many, many real events to fire the imagination.
SFB is a game about a TV show, with limited history, and very few actually televised battles to fire the imagination. Any other scenarios created for the system have the disadvantage that they didn't "really happen". They may make a good game, but they won't appeal on the same level.
Warhammer is a game with quite a lot of made-up history, but was always designed as a DYO game. The costs of miniatures make it prohibitive to create scenarios with an OB and then expect the players to go invest in that OB in order to play them. Instead, most Warhammer "scenarios" are actually more like SASL "missions"; they include information on how to set up a random board, what each side's goal is, and any SSRs that apply. Then you can play the battle with whatever army you've already invested your thousands of dollars in.
Battletech also has quite a lot of made-up history, with a number of scenario books based on that history. But most people I know play it with miniatures, and thus run into the same problem as with Warhammer. For reasons I've never understood, the scenarios also tend to be uniformly quite large. If every published ASL scenario was 10 turns and had you running a battalion, then DYO ASL would be my game of choice, too!
In short, we're wedded to scenarios because we CAN be!!!
John