I don’t know about cooling specifically, but for most space-bound equipment, it’s all about SWaP: space (volume of the equipment on the rocket), weight (more weight means more propellant needed), and power (you only get so many watts out of a given solar panel, and if you want more, go back to step one).
So they presumably found that 6x6 is an acceptable configuration for the available resources.
And that doesn’t even touch on the equipment itself, like _WC did. More pixels means more complexity means more engineering, testing, etc.