Well, sometimes it's that, but it also has a lot to do with supplies of components drying up. I think the Subway was designed almost a decade ago and the only reason the last production run was possible was because the developer of another (unreleased/incomplete) Amiga USB card sold his out-of-production parts to E3B.
We'd all love to see more availability of Amiga stuff. It holds its value incredibly well, so there's no risk of a manufacturer eating the costs of unsold product from a larger production run. The challenge is that the return on investment is years long, so developers/dealers may not be able to wait that long to recoup costs, or may not have the cash on hand to order a larger quantity in the first place.
Group buys, then, like so many other websites do for runs of things that cost too much to do one-off. Subway may be 10 years old, but the deneb isn't, or the xsurf, or fastATA, or the a1200 cards that Jens did. All viable, all sellable.
Also, it holds its value incredibly well because of scarcity. Check what people are selling Denebs for not even a year after Vesalia and AmigaKit sold out. A 25 dollar SOC can put a USB chip on their board, and for another 10, a NIC. Perhaps they should choose some cheaper, more available hardware to begin with.
