Because there were so many of them (13 sessions if you include Babes Blues which was released in the late 80s), it would make for one big ass box that wouldn't exactly be cheap, especially since its almost a given that a full blown Mosaic box will include unissued tunes, making it even bigger.
The Selects are a lower price point and I would assume that Michael expects more people to buy one or two of the Selects and not all three; better to give them the chance to sample than to ask them to pay $100+ for the whole shebang, especially when, and even the former GHF will admit this, there isn't a lot of variety to the recordings.
But that would apply to many of the bigger (7, 8, 9... discs) Mosaic boxes, Dan. And this hasn´t been the policy so far.