The reason it hasn't sold as well is because everyone wants a Pro for the features, so you end up having to choose between the better size and the better build materials and features.Why would they do that?
The 12Mini hasn’t sold very well, why on earth would they make an even more expensive and complicated version of it that’ll sell even less?
I wanted the Dolby Vision recording at 60 FPS, stainless steel frame, and telephoto lens; but I ended up deciding the that size of the mini made it still worth compromising those other features. Many people I've talked to who bought the 12 Pro wanted the mini's size, but didn't want to forego the "Pro" features and ended up going with the Pro.
If they made a mini Pro and stopped trying to force people to believe that a smaller phone has to be a cheaper phone, they would've had no problem selling more minis. As someone else said earlier today, nobody is buying the mini to save $100 over the iPhone 12 -- they'll just pay the extra $4/mo over 24 months to get the size they actually want. But a lot of people are foregoing the 12 mini because it lacks the features that they would be giving up coming from the 11 Pro or earlier, like the telephoto lens.