Thanks for all the responses. I think the best point is, that's just the way it’s always been done, and manufacturers are used to it. “Stick to your knitting” is a good point, but I don’t think that would be a disadvantage with this industry.
I didn't realize that DSD was uncommon (The CPG company I work for is the only one I've ever worked for). What didn't occur to me when I posed this question is that our main competition uses distributors, while we use DSD and only use distributors where the economies of scale don't justify us using our own trucks--places where we wouldn't sell enough product to fill up a truck, but a distributor, carrying products from multiple manufacturers, could.
The main reason we use DSD is because we have more incentive to sell our own product than a distributor does. See, a distributor doesn't really care what he sells, as long as he sells a lot of something. You have to rely on "pull" from the consumer/retailer. When you sell it yourself, it is easier for you to "push" it.
I know the hassle of collecting from many small retailers, because I work closely with the people who do. But that's how my company grew, so I guess were used to it, while in the gun industry, they all grew using distributors, because that just happened to be the way things already were, so it might be hard to change.
Even though I'm not being an expert in this, I still think gun makers would do better "pushing" their product, since firearms are a very controlled and almost "niche" industry, that can't easily advertise to a mass of consumers and thus help create pull. I mean, a lot of shooters don’t ever read gun magazines, or hang out at the range, or gun shops all that much. They don’t really know what’s out there; they just bought whatever was convenient at the time. I have to spend a lot of time researching to find out what’s good and what’s out there with guns. It’s kind of like that with guitars or musical instruments—does anyone know if that industry uses DSD or distributors?
I also find distributors to be annoying as a consumer. Me to gun store clerk: "Springfield has product X on their website, could you order it". Clerk: "Uh, I only ever talk to the distributor, and they don't list it." Having a distributor just creates a level of bureaucracy between the end-user and the maker.