The release schedule was crap, as well as the 12 mini being an objectively bad phone.

Spring 2020, they released the iPhone SE 2020, 4 years after the previous iPhone SE. This satiated a lot of the demand for people holding out for a smaller phone. Then came the surprise with 12 mini in September of 2020, except the battery life and performance sucked, garnering bad reviews.

Then, finally in September 2021, they released the 13 mini, an objectively good, smaller phone. But over the previous 18 months, a lot of the buyers for the 13 mini had already bought the 2020 SE or were burned by the 12 mini.