To unlock rk you had to have a lvl 55 character. So when cata came out a player would have to level through vanilla and bc, bc again and wrath to have a Dk. Same with anyone returning in bfa and wanting to play a Dh.
Allied races at best are a trophy for completing certain content. Most of an expansions features you have to invest time in to see a reward.
I'm sorry you have to play the game to get rewards.