play, or re-play WC3:TFT and you should have your answer as to why Sylvanas was fit to lead the Forsaken at the beginning and why Calia hasn't earned it.
I don't think anyone pretends that synlvanas retains that status though, she's clearly unfit to lead anything now. But when the forsaken were formed? She was the leader they needed and shared their pain and trials. She was even there with them when the Alliance shunned them, giving them their name. Calia remained in hiding rather than reaching out to her people.
She could be a leader someday, many of the forsaken (as read in BTS) still revere the Menethil line showing quite clearly that they separate Arthas from his sister. If she did it now though It would, in my opinion, feel rushed and disjointed.