That looks like planned obsolescence to me. A customer with 1TB will likely want to upgrade sooner than a customer with 8TB, so this pricing strategy discourages people from buying the more future-proof options. Other SSDs on the market tend to be cheaper per TB as you go up in size, but Apple's seem to be completely backwards (and obscenely overpriced, of course).
Or am I being too cynical here, and this is just that famous "luxury tax"?