Musar על יבמות 127:9

As long as Israel was in its own land, G–d yearned for our prayers, and that is why the patriarchs were people whose prayers are mentioned and from whom we learn the importance and efficacy of prayer. G–d even withheld basic needs such as children from them in order to "provoke" them into prayer. All this was true only as long as we lived in the land of Israel, a land which is under G–d's constant direct supervision. Since the other countries are supervised by G–d's שרים, angels appointed to look after the needs of the respective Gentile nations, why would G–d be interested in our prayers. Would they not become mixed in with alien elements, i.e. the respective שר supervising the nations that the Jews live in whose prayers are sent heavenwards? This is why the advice to the Jewish people while in exile is to send their prayers via ארץ ישראל, from there to Jerusalem; from there to the site of the former Sanctuary; from there to the site of the former Holy of Holies, the Inner Sanctum. From there the prayers are dispatched to שער השמים, as our sages have described this process. ...