Co-op campaign as a recruit?

Why can’t I join co-op campaign as a recruit in MM?

I actually just leveled up to Private, and I still can’t join.

It’s flashing on the screen “One or more players have a Grade too low for this playlist.”

This doesn’t make any sense to me, why are there “grade” or “rank” limitations for this playlist? Sure, I could just hop onto my other account, but I felt like starting all over, but playing co-op online with others. I know it wouldn’t be in order, but I already know the story.

I dunno - it’s weird, and confusing, and kinda frustrating - mostly because I can’t think of a logical reason why it’s like that.