It's clear Cover has 1 year contracts and everyone goes through a re-negotiation process that covers how much streaming they do. Ayame barely streams, but is in nearly every 3D Live and does a lot of promo work. My assumption has been that Cover did offer Sana something like that contract, but the combination of primary work and health made her decide graduating was the best option, for her. Only makes the fact she dropped probably the best original HoloPro song not long before that sadder.
to @God's Strongest Sanalite