I can think of 2 reasons why Apple did this:
- chip shortage: they are worried about yields on the A15 and they to use the "perfect" chips for their iPhones and anything less than perfect can go into the iPad Mini, which isn't their primary cash cow
- battery life: upon testing, they found out that the battery life suffers on the iPad Mini when a "normal" A15 is put into the iPad Mini
Battery life wouldn’t be the reasons. It changes linearly with frequency, so it would only be a handful of % difference (less, given that most of the time the processor wouldn‘t be running all cores at full frequency).