I think the onward march of phone sizes into the 6" range is cannibalizing the market for small tablets, and 9-10" for tablets is the remaining sweet spot between phones and laptops.
Huge tablets aren't exactly an unexplored niche; the Galaxy Note Pro 12.2 and Microsoft Surface, and even a few bigger tablets, do exist.
I guess I unintentionally excluded the Surface Pro 3 since it's so much more than just a tablet.