Apple says they stopped producing minis because they didn't sell. It seems they sold relatively better than the Air, and pretty much everyone I know who still uses a device of "13" or earlier generation, is on a mini. That's about 5 people just in my social circle still on a 13 Mini, and 0 people on any other non-Mini 13th or older generation. I reckon that's the real reason they stopped making them, people who use them, are willing to stay with their phones for much longer periods. Could also be that they break less due to being smaller.