The Apple Intelligence page specifically says iPhone 15 Pro for AI functionality. I think they're trying to keep things on device as much as possible, which can't be done on older devices. The cloud servers are only used as a last resort on already supported models.Are you sure? It was hinted that Apple Intelligence runs onboard on A17 pro chip or Apple Silicon, or on private Apple severs for other devices, presumably slower.
Maybe that isn't part of it.