There are two points of supply. The breeder, and the distributor (seed bank). The breeder, in our industry, is pretty much improvising what strains he wants to release, and doesn't do a great job notifying their seed banks of what they'll have coming up (partially because they don't know). So, seed banks just flat out don't know if a strain will ever be made again. How could they? They're just middle men. Breeders themselves don't even really know if a line will ever be re-visited. I mean, they're just people. Some of them have pollen stored in a freezer from a past project, or seeds in the vault, and some of them don't. Sometimes they get the itch to rework a line, other times the latest hype strains get their attention.
Work with what you can get your hands on right now. Stay on the mailing list 'just in case', but if somethings out of stock, don't try to fit it into your timeline at all. Treat it like it's gone for good, and work with what you can get your hands on. And you can try other seed bank sites, or strainly, or trading with members of forums.