Benjamin Mayo, on bzamayo.com:
Apple doesn’t really care for the smaller size, they want the price points. By 2022, the components of the 6.1-inch model will have gone down and Apple will be able to sell it as the cheap flagship again.
Bit late, but I think Benjamin nails it: Apple brought the mini size back because the flagship price went to high.
That said, both my wife and I love our 12 Minis.