To expand a little on what Cilia said above, Ishgard used to be an ally to the three Eorzean city states (even going so far as to fight to defend Gridania in the past) but withdrew and became very isolationist shortly before Garlemald began their invasion. We still don't know the actual cause of this change of policy but their neutrality is presumably what spared them an invasion of their own.

Ishgard is opening its gates now partly out of desperation, but also because Aymeric (and to a lesser extent Haurchefant) has proven to have a great deal of influence in the city. He has negotiated on our behalf to allow the WoL and their allies shelter out of recognition for all that we've done to help him and Ishgard to date. Depending on how the Ascian influence of the Archbishop plays out I won't be surprised if Aymeric ends up taking full control of Ishgard by the end of 3.0.