Because they kneecapped the mini and marketed it poorly from day one. If Apple wanted to sell a pile of minis they'd market the size as the "perfect" size for most people (it is) and put a smidge larger battery in it to get it 20+ hrs of playback.The mini sold less than 5% of all iPhones, meaning that they were easily outsold by older models too. A mini Pro model would likely sell even worse.
Almost nobody except some Macrumors diehards want minis.
The problem is the profit margin is probably way smaller than a "Pro" model. Let's not even get started on how terrible the 13 color palette was - you can have blue or blue, Dirty White, Red, or (3 months later) Generic Green. Apple held onto "small" screens for a long time and marketed it well. 5.4" edge to edge with great pixel density is not small for daily tasks. Throw in 120hz and a real camera and you'd have a device you could easily market to the masses.
Everyone who has held my 13 mini has commented on how much they like the size over their 6.1" phones. I don't buy that people wouldn't purchase it, but I do believe that Apple didn't want to sell the mini in large quantities because of the margins.