I know in the US at least, it was Amazon exclusive for a physical copy.
I'm guessing that it wasn't expected to sell well physically and Amazon maybe only ordered a low amount of copies? I know it was still in stock until relatively recently, so maybe that was that whenever they eventually just sold out.
In any case, have had my physical copy since day 1 and not gonna let it go. It did get minorly damaged when it shipped to me though -- was just in bubble-padded manilla envelope, so the front of the case got crushed a tiny bit and it looks like someone ran their fingernail across the front a bit, not enough to tear but enough to warp the plastic a little.