I imagine it’s due to brown characters universally selling worse than the red/blue/green ones. Make their personality/story involvement more interesting and people will be that much more inclined to buy them.
I have always liked the po-matoran more than the other tribes, just because they were honest hard workers and felt the most life-like of the matoran. They worked, complained, and played sports like humans. I also have always loved the color scheme… I just wish that pohatu was brown and old dark grey.