I don't want to speak out of turn, so my disclaimer is this - I'm only a long-time customer, and I know nothing of what goes on behind-the-scenes. That said, I can't recall a time that these collect-a-piece Collabs have been offered up for sale in their entirety, like when the store brings back collabs you might have missed during the year, and you can earn them by spending $20 apiece or whatever. The ones that are brought back are the ones offered up as a whole in store incentives as freebies.
What I have seen done is some designers will take their contribution to the collab - be it a paper pack, or a set of word art, or a mini-kit - and add it as a product for purchase. Not all of them do it, but sometimes it happens. For example, the "Shiny Happy People" collect-a-kit - you can find Brook and Em's templates, Nat & Jennifer & Tracie & Laura's mini-kits, the adorable little people stickers by Lliella Designs and papers by Erica. That's just a fraction of the overall kit that was available to collect, but those are the pieces you can purchase if you missed them.
Similarly, you can find pieces from the "Daily Dose" collect-a-kit in the shop. On the other hand, I haven't seen any pieces from "That Was A Year" or "20/20" available for purchase.
I guess it just depends on what they feel like adding, and maybe customer requests? So it can't hurt to ask for specific pieces from specific designers, I'm sure.
