Plus her vast psionic powers make her the perfect avatar. Jeen knocked down Galactus with her siphoning powers. In the future she took down for Silver Surfers and Galactus.
I think Jean was the prime host sue not only her powers, but her empathy and compassion. Her character. She sacrificed herself to save her friends. So she was the best person to save the universe from crazy D’Ken and the M’Kraan Crystal. It’s a combination of power and character.
The Phoenix does not want Madelyne, who has poor character.