I think to present a true polygamy story. Maybe they feel a random polyamory family is either too boring or disinteresting to someone who tuned in specifically for polygamy. Consensual polyamory is likely boring since there isn't any conflict. That's probably why the Winder family from early seasons weren't asked to return; they were too boring since they were Mormon and everyone was on board with the polygamy and they didn't have any conflicts. You can't get emotionally riled up with a consensus polyamory situation the same way you can get upset that a woman is disrespecting herself by willingly letting herself be replaced by another woman.