Because, frankly, I don't miss it. Kindle gives me more bang for the buck (portability, batterylife, ... WAY cheaper...). We have an entire Apple Ecosystem. For casual browsing, I use my iPhone, for mobile work I use my MBP. For reading I use my kindle - they've become extremely good. When I started out, they were too slow for my taste, but they've really caught up.
A Pro Mini would be an INSTANT buy for me, though, just because I could do so much more with it. I love the idea, style and versatility of the new Pros, I LOVE the Pencil 2, but it's just too big for me. And I'm not going to put (a lot!) of money into old designs and castrated features for no other reason than "just because" and to push people to buy the more expensive versions (take a hint, Apple: I'm more than willing to spend $$$ on a small-sized Pro. It's not about having a cheap entry-level iPad but about a SMALL version for SMALL people).