johnsemlak said:
Is there any particular reason why other companies don't do the same?
$$$. The upfront costs for publishing a book are big. You have to pay for art, freelance writers, printing, marketing, etc all way before you make a penny. Adding to those up front costs before you even make a sale is hard to do, and is risky if you don't know if you'll recover any of the added extra expense. It seems like most publishers simply don't see it as worth the cash (or they don't have the cash, like me

). Kenzer may be the smart ones here, if they're doing well with Kalamar. Perhaps other publishers would do better if they spent the money.
But it's hard to predict "I'll get X more sales by spending Y amount of cash so that I can put the D&D logo on this product, and therefore I'll make more money". Kenzer might be the only company with sales data that can be used to scientifically determine if it's worth it. And even then, their data can't be complete, because they didn't try to sell Kalamar without the D&D logo in the same market, so it still requires an educated guess under the best of circumstances.
The other reason might be WOTC's willingness to sell the use of the logo. They're not going to want you to use it if you are competing with them directly (selling books similar to the books they are publishing, the Arms and Equipment Guide comes to mind) , and may not want to sell to companies who publish products that they consider substandard in quality. For example, if you consistantly publish products that have horrible art or poor bindings, they probably won't want their trademark on it. So some publishers may have approached them and were turned down.
My guess is that for certain types of product series like a big campaign book with sourcebooks, like Kalamar, it would be worth it. I don't think it would be wise to purchase a license just for one book. You could probably get a better deal by purchasing it for multiple books. All IMO of course.