Blizzard doesn’t want premade raids in unrated bgs.
https://www.engadget.com/2012-12-05-blizzard-takes-a-stand-against-pre-mades-battleground-groups.html
Premade raids vs. pugs are one-sided games that drive players away from PvP.
If you want to PvP in a raid group, Blizzard wants you to do rated battlegrounds or war games.