The mini was released six months after the SE2 was released, which many SE1 owners holding out for a new smaller iPhone had already migrated to. Then Apple presented the mini in a joke-y manner in the keynote, didn’t do any publicity afterwards, and neither did the carriers (because the mini was lower-margin for them I presume). Lastly, the iPhone 12 gen had particularly bad battery life, which also hurt the mini. With today’s battery improvements, it would be much better (as demonstrated by the iPhone Air’s good battery life). Apple could at least have taken the option to make the “e” line mini-sized.