TL;DR
My assumption is it's a combination of carriers doing away with subsidized contracts, 3.5/4" screen lovers holding on to their phones, maturity of the product, and the fact that the 6S really, honestly is not much different from the 6. ""To Me"", it is not noticeably faster, not noticeably better performing, photos looks the same give same aperture, 3D Touch is laughable, and the phones look identical. I would think even enthusiasts like us in the forums have had an inner struggle as to whether the S versions are worth the cost of an upgrade.
(I do find the jump from the 6 Plus to the 6S Plus a bit more worthy given the impact of 2gb ram to that particular model)