What is the airspeed of an unladen Swellow?
My post was a joke about the sheer number of hat/costume Pikachu that have appeared in Pokemon Go.In this case it's probably so they can sell merch featuring THIS Pikachu alongside the generic Pikachu design. It's like having two popular Pokemon in one.
They did the same thing with Coco, where they gave Pikachu a pith helmet just so they could put out a bunch of pith helmet Pikachu stuff as movie merch.
They also had a special Jungle Explorer Pikachu available in Pokemon Go as a Coco tie-in.