I think there are several potential reasons for this:
- Apple will have to put in the engineering effort to redesigning the chip for next year's non Pro models as the N3B platform will give way to the N3E platform and the designs don't transfer directly. Given this, why spend the time re-engineering a last gen product?
- Apple saw non-Pro iPhone 14 sales fell off a cliff last year (with not everyone moving to the Pro models) and think putting in the 'latest' chip will boost sales