I suspect it’s because the iPhone mini also makes Apple the least amount of money, margins wise. This is a consequence of Apple positioning larger screens as being more premium and costing more. The flip side is that smaller screens must also cost less.
Even if the plus iPhone sells the same quantity as the mini, it automatically wins by virtue of costing more (and consequently making Apple more money).
I understand this is why Apple favours the plus sized phones, and it is my point exactly. Even though there is a reasonable amount of demand for a Mini phone, Apple doesn't care as they can make more off larger phones. It's a cycle of no small phones to buy = no demand. I just wish Apple would at least give the option, after all they are supposedly the company of user experience.