I believe the Seven Mothers Cult was deliberately engineered as an "entry-level initiation" to Lunar Society. Remember, the Red Goddess is a massively evangelising religion. It isn't like most of the other cults because it is somewhat akin to Christianity as a de novo god and religion whose followers are interested in aggressively proselytising.
And in Glorantha, where your cult teaches you life skills, an entrance path for the new city-based faith of Lunarism is important. The Mothers focus on the basics of indoctrination, the seven virtues of Sedenya, and basic learnin' about what it means to join the Lunar civilisation. For a lot of people, that's all they need, even inside the Empire proper. They aren't capable, interested, or trusted in diving deeply into a specific cult. If they are, they'll probably choose one of the Mothers and be selected by the priests.