Plenty of people use Amsoil. Maybe not a household name, but most every auto enthusiast has heard of it. Most every snowmobiler, biker, RC'er, garden professional, etc has heard of Amsoil as well.
Mostly the reason for people sticking with brands is gut feel. Some people don't readily change from what their fathers used. Other folks do seek the new, and in general people can order oil without problems.