The biggest problem with the iPhone mini is tied to upgrade cycles.

People always they want an iPhone mini, but current upgrade cycles for most people is about 2-3 years. So Apple finally makes one, but two-thirds of the people who said they wanted one are at the wrong spot in the adoption curve, so they don't buy one.

Second biggest problem with the mini is that it's always short on features and battery capacity.

Which again means that people who said they wanted one don't buy one.

And off the market it goes.