This should be because isa_all includes more categories than isa_class and isa_instance, and thus can capture more semantic information.
although All and Alladmin include even more categories, they introduce many categories which are unsuitable to model the topics of a news article or blog text, such as the two categories mentioned in Section 3.3, "people by status" which is not in an is-a relation and "Wikipedia editing guidelines" which is used for encyclopedia management.