Lets look at it objectively shall we.
Calia:
Has no real connection to the forsaken, except her name
Has no leadership skills
Has no tactical knowledge
Has no diplomatic experience
Is most likely a naaru puppet
Is basically just human, suffering no characteristically drawbacks of undeath, meaning she simply can't understand those who suffer from that.
Hasn't been part of the forsaken for pretty much their entire existence
Is incredibly naive
She just does not fit. She lacks basic qualifications for any kind of leadership position.