Camera blocks take internal volume. Smaller phones have less internal volume to spend - while still having to pack all the non-negotiables like the modem and the SoC into it. Something's got to give. And no one want that something to be the battery life going down to 6 hours.
Volume constraints are bad enough for "normal" models. "Minis" have it way worse.
Volume constraints are bad enough for "normal" models. "Minis" have it way worse.