A non-linear cost for a small fraction of the phone. A slightly larger PCB with two more layers is about twice the price for the PCB, more antennas is a couple dollars, a one lane PCIe switch is a few dollars if that's cheaper than upgrading your SOC. And if it shrinks your battery 10% you save money on that part.

I blame niche-ness far more than the actual design and production costs.

Edit: fixed the pcb wording