You're right, Brooks doesn't sponsor anyone. They are a small company and probably spend their promotional/ advertising money on print ads. Mizuno and Fila are in the same boat, except that they do have athletes in Europe, Asia, and Africa.
Reebok will be adding new athletes in the coming months. There's a new team in place and they will focus on signing a couple of good athletes. Remember, they were very, very close to signing Webb.
Besides Drossin, Asics does have some pretty good young athletes. Checking the Running USA website, they sponsor Billy Herman, Sara Wells, and Kristen Nicolini. I saw Dan Wilson and Janelle Krause wearing their uniform at the Midnight Run. I think Amy Yoder is also running for them.
My guess is all these companies, minus Reebok, have limited promotional/advertising money. Unlike Nike, or even adidas to some extent, they have decided to focus their efforts on other things or just smaller groups of athletes. I'm not sure it's their responsibility to provide assistance to every decent athlete out there. Just because it has always been that way, doesn't mean it's the right way.