Why not add All For One: mirror mode along with the original?

I'm so happy that All for One is finally here (and get free mastery points hurr), but why not add Mirror Mode as well? I really liked Mirror Mode since all it really depended on was skill, instead of who got the better match-up sometimes. It was also really intense and quick paced on the Howling Abyss, which made for quick paced carnage.