New dialogue from Thalyssra , Shandris and Azshara in 8.2 reveals this, but didnt Chronicles say it was shielded to save the city from demons and sundering instead of her order joining the High Priestess and the others because Elisande didn't believe the resistance could succeed?
There can be several reasons to do a thing, but the main reason is the most important, we know Tyrande wanted Elisande's group to join the push to Zin'Azshari after they had sealed the second portal and presumably created the nightwell. We know the Queen at this point had cut off the empire and capital from accessing the Well of Eternity (read WotA trilogy), naturally the nightwell would have been a powerful tool.
The conversation has Shandris accuse the highborne of Suramar (whom Tha,yara represents) from abandoning the rest of them and Kalimdor to burn to wall up, but she later agrees that the shield was to keep Azshara from the Pillars of Creation - when she says I know..
This new information means or implies their was much going on