Of course, Ariel didn't know how she turned out to be in the original story after the fact that the Saintess that was chosen turned out to be Serenia.
She thought that she had the utmost backing of the empire, but it was probably just a little experiment by the Emperor, not meaning much of it. Other theories might suggest to Caelum manipulating some strings behind the back to shove Glazelle away from the position.
If Serenia, who he was privately close with stepped up to the position instead of Glazelle, it would be akin to forming a separate connection with the Holy Land. Not for his father Emperor, but rather himself.
Glazelle did have strong powers.
However, she was too occupied with the thought of being clean and above the others. Humility was not a word in her dictionary. She was born and raised as a princess, having manners drilled down to her and the understanding of a royalty imprinted on her heart.