I’m gonna share my thoughts, I’m sure many people will disagree, but this is opinion about why the mini failed in sales.
First of all, I personally find the iPhone mini screen too small, specifically too narrow compared to the aspect ratio of the mini, and the width of the iPhone 8. I’m not saying that I prefer the width of the regular 6.1 inch iPhone, because that’s a bit too wide. However, I think making it a tiny bit bigger, in the realm of 5.8 inches, would have made it the perfect sized iPhone for me. Emphasis on for me.
The second cause I think affected the iPhone mini sales is the naming. I think the mini name would have made people think it was less worth, and naming it the 5.4” iPhone and 6.1” inch iPhone would have had less impact on sales.
But still, I think a bigger size like a 5.6 or 5.8” would have been perfect. While trying the iPhone on the stores, I’ve heard so many times people trying the mini in front of me say ”no, not this one, this is too small”, that honestly I think they went too far miniaturizing the phone, and they made a niche pretty happy, but they lost a big chunk of the mainstream sales.